My two cents regarding the combat system/builds.
Bugs:
1. Howl (Elena Wolf Form) lasts 1 turn instead of 2, contradicting the tool tip.
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Observations:
1. Restricting max to +5 of the lowest skill is ultra annoying.
Not only MC has 2 more stats (charisma, banter) to level up compared to his companions, so he unlocks skills slower in comparison, but some stats are practically useless for some classes. For example Dexterity isn't useful at all for Marcus monster form. It's not used for skill unlocks and it's better for him to focus on increasing mitigation over evasion.
2. Single target damage scales poorly
Against Zanarak (48 armor), Venomous Strike (227-376 damage on tool tip), does 85-105~ damage. Other offensive abilities get similar decreases in output. The bigger problem here is that it's hard to calculate how much damage an ability is expected to do. Expending 50 energy to deal so little damage, and without understanding how the formula works, is kinda pointless, making defensive builds better overall,
3. I don't understand how poison/burn damage is calculated
MC's poison from Venomous Strike deals 20 damage per tick. Erika's burn (Eye for an Eye) only deals 8 per tick and Marcus' burn from Fiery Counter 16. I highly doubt that it's the difference in stats so it's either a hidden calculation, or each skill has it's own effect that is hidden to the user.
4. I don't understand accuracy
Despite having more hit than the enemy's dodge, adding the Howl buff on top, I still miss more than I'd expect. For example, Zanarak has 10 dodge, the weakest party member has 24 hit chance, and I still miss 2-3 times during the fight. Is there a cap on hit chance? I'm certain that Accuracy Buffs affect hit chances dramatically based on experience, but I can't possibly calculate the effect and it doesn't seem like 100% hit chance is attainable.
5. Provoke is kinda overpowered
It's not rare for taunts/provokes in turn based games to be overpowered but Marcus with Myu support basically trivializes boss fights. The reason is that not only Marcus is naturally bulky, but provoke forces the enemy to use the basic attack, which may be orders of magnitude weaker than whatever ability the boss was gonna use.
The tactic is something like:
1. Marcus uses Fiery Counter -> Myu uses Defend -> Rest of team executes whatever offensive setup
2. Repeat 1
3. Marcus uses Defend -> Myu uses Slime Recharge on Marcus -> Marcus uses Fiery Counter -> Rest of team continues attacking
4. Go to 1
Basically Fiery Counter doesn't have an accuracy check, so against boss type single fights like Zanarak, as long as you manage energy regeneration well, you can't really lose this fight. And in worse case scenario where Marcus goes out of energy, just spam some potions to sustain incoming damage for a turn or two.
Naturally I don't think that nerfing Provoke in general is the best approach. But at the very least, hard boss fights should have minions and perhaps it's a good idea to introduce accuracy checks to Fiery Counter in particular.
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Builds / Combos:
I won't go into too much detail here, but I think that these are the most useful skills per character in my opinion
MC:
Despite what you may be used to in other games, MC is not the strongest character. His Parasite form is geared toward's damage dealing but his actual capabilities are kinda meh. His support abilities are outshined by Myu and Elena/Erika. Marcus is a better tank. So MC is a jack of all trades with some minor damage dealing specialization. You will easily notice that his damage, especially the single target one, is really low without heavy support from his team mates.
However do note that MC gets some potentially strong abilities later when he reaches strength 20 and it's impossible to get it that high as of current content.
Human Form:
Flailing Strikes - AoE abilities are absolutely crucial when dealing with many weak enemies. It has good scaling for an AoE attack. Compared to Whirlwind of Steel it's strictly worse, but the latter requires 20 Strength which is impossible to get currently.
Warrior Stance - AoE provoke + counterattack with some minor aoe damage on top. It's a good skill that can be easily combo'd with Myu's recharge (Flailing -> Recharge -> Warrior Stance). The issue is accuracy, so make sure to always buff MC's accuracy before using this skill, since it becomes much worse with each miss.
Guardian's Shield - This skill is good for the earlygame, before provokes become commonplace. Generally speaking you want the enemies to focus on a single durable target, so you can buff/support/heal him instead of the whole party.
Parasite Form:
Acidic Burst - One of the strongest AoE attacks damage wise, and at 4/4 it also causes stuns at 60% chance. It has potential uses against single target enemies thanks to the stun chance, but it's more of a desperation/energy management thing, rather than a consistent stunlocking ability.
Venomous Strike - Thanks to the poison effect, this is a quite consistent offensive ability. Without team mate support, it's expected to outperform the other offensive abilities
Razor Slash - I don't like this ability that much, but the armor debuff can be effective against certain enemies. The thing is, against enemies whom the debuff makes a difference, MC's participation and damage output optimization doesn't really make much difference. It's usually up to the other team members to manage the fight.
Perfect Organism - This ability works well in tandem with Myu's recharge, potentially turning some tactics perennial. However so far I didn't really have to use it because Myu is generally good enough to sustain energy expenditure for more than enough turns.
Marcus:
Marcus has certain advantages over the MC. For one, since he doesn't have Charisma and Barter, he has higher combat stats than MC. So his humanoid form is technically better than MC's assuming similar gear. His parasite form is specialized towards defense and he is practically unkillable past a certain point. While it's hard to test, I think it's possible for Parasite Marcus to solo anything in the game, though it will take a long time.
Human Form:
Flailing Strikes - Same as MC
Warrior Stance - Same as MC
Guardian's Shield - Same as MC
Obliterating Blow - Marcus can get enough Strength to buy this before the end of current content. Don't expect it to "destroy" single targets, especially without support but it's better than the other 2 based on the few observations I could make.
Parasite Form:
Fiery Counter - Provokes are strong, especially when they don't even need an accuracy check. Parasite Marcus is the tankiest character and the energy cost is very fair. So he can trivialise fights with big bosses that depend on some ability to do serious harm.
Infernal Parasite - Due to how hit points work, this ability pretty much heals 100% of Marcus' health bar each tick so it's a perfect self heal. Also the energy regeneration debuff can find some application, especially since it's AoE. Energy cost is good, but try to avoid using it as much as possible in boss fights. Fiery Counter + potions is a better solution in the majority of cases.
Scorched Earth - It's always good to have some AoE support. This one is average, but it has some potential for the future when Marcus will unlock Eternal Fire.
Elena:
She is one of those characters that has way too much stuff, and most of it is.. kinda useless. However she shines as a support, buffing party's offense. Both her forms are capable of buffing accuracy, which is very important because missing sucks in this game and it tends to have a snowball effect, where missing a key ability makes you take too much damage from the enemy, which in turn forces you to expend resources managing it and it goes on and on, turning a trivial fight to an annoying/dangerous one.
Humanoid form:
Howl of War - Best offensive buff in the game. It shines against multiple enemies where the party uses AoE tactics.
Acrobatic Maneuvers - Accuracy + Dodge isn't necessarily a good combo, but it only costs 50 energy. So in drawn-out fights, it may be better than Howl of War.
Wolf form:
Howl - Similar to Acrobatic Maneuvers. Armor seems to me a bit better than dodge as a defensive buff, but again, the accuracy buff is the big draw here.
Snarl - One of the better provokes, quite good against many enemies. Against single strong enemies it's better to let Marcus deal with it. However do note that accuracy is even more important in AoE fights, so Snarl has quite high opportunity cost compared to Howl.
Feral Strikes - Having some extra AoE support may be useful during the siege, because the ghouls regenerate and it takes a while till Marcus joins the party.
Myu:
She has a ton of support moves but her best ability is Slime Recharge. It's so strong that you should refrain from using Myu for other types of play, like offensive abilities or general defensive support, without a good reason.
Slime Recharge - By far her best ability. It allows other character's expensive overpowered abilities to be used more frequently. The extra turn part is ok, but my recommendation is to use it with the "Defend" ability in order to boost the energy regenerated from 30% to 60%.
Expand Shape - A very strong defensive buff. Works well with counterattack tactics
Corrosive Cascade - The main draw is that it debuffs damage taken, making all the subsequent AoE attacks stronger.
Erika:
If Elena is a good offensive support, Erika is the same, but on steroids. She has the same issue of having too many abilities which aren't that effective, but the good ones are absolutely bonkers. Just buffing party's offensive would be good enough, but adding the AoE stun with perfect accuracy and dot damage on top.. makes her absolutely dominant with Myu's support.
Eye for an Eye - This is an extremely powerful ability that stuns everyone without an accuracy check. The HP sacrifice is trivial, as every enemy is stunned for 1 turn, and potions are kinda cheap/easy to come by.
Old Gods Bargain - This is similar to Elena's Howl of War. It exchanges a stronger effect for a defensive debuff.
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Party Compositions:
Early game - MC + Marcus
Early on it's just MC and Marcus for the most part. Gearing Marcus towards defense and MC toward's offense is the natural choice. The important skills are Flailing Strikes / Acidic Burst for MC, Flailing Counter / Guardian's Shield / Scorched Earth / Fiery Counter for Marcus. Eventually you want one of them to get Warrior Stance.
Early mid game - MC + Marcus + Elena
Pretty much the same, but now the party has offensive buff support. Generally speaking, you want to use Elena's offensive buff, then either go full offensive (usually against AoE fights), or play around Warrior Stance / Marcus' Fiery Counter (depending on if you fight on human or parasite form)
Late mid game - MC + Marcus + Elena + Myu
Myu offers excellent support both when fighting in human or parasite form. Her AoE damage ability increases the party's damage output and her Slime Recharge allows Marcus to deal with pretty much any boss fight consistently. Elena's role remains the same.
Siege - MC + Marcus + Myu + Erika
Erika is an obvious improvement over Elena. Not only she can mimic Howl of War with her own Old God's Bargain, but she can also support Marcus through stunning the enemy team with Eye for an Eye, extending the basic Slime Rechage + Fiery Counter tactic against Zanarak.
Early Second Act - MC + Marcus + Ves + Kiara
Since the game pretty much stops when reaching Hamun, it's hard to say how effective the team when it can only be tested against the travelling encounters.
Ves' Berserker Time isn't that great because it doesn't boost accuracy and her other abilities overlap with Marcus. Her damage dealing is weak. So far I can't see how she can substitute for any other member. Crush is probably her best offensive ability against single strong enemies (though the energy draining effect is kinda weak), and her cookie cutter is WarCry which is an AoE provoke with defensive buff, but I can't find a true niche for her.
Kiara on the other hand seems more interesting. Song of Dark Bargains is the same as Berserker Time. Eagle Eye seems ok'ish, being an expensive buff but debuffing an enemy at the same time. If Song of Madness turns out to be as potent as I hope, then she may be good enough to permanently take over Elena's role, though Erika should be superior.
Still this party needs a lot more testing. I want to see how Ves compares to Marcus overall. I don't see her ever performing better against a single big boss, but perhaps she is better in more complex fights.
Edit:
Tried to beat the Kraken with MC + Marcus + Ves + Kiara. It's definitely doable as long as you cycle through War Cry / Fiery Counter. Drops a "Mysterious Gem" but I don't know what to do with it.
Kiara:
Kiara's Eagle Eye is very strong both as a buff and as a boss debuff, but it also costs a ton of Energy which is a huge negative. Myu should be able to keep her afloat in extended fights (once we get her back at least), but competing for Myu's recharge is a bad thing tbh. I didn't like her monster form performance that much though. With that playstyle it's almost impossible to have spare energy to use any other skills. I wouldn't say that Kiara is worse than Elena's humanoid form. Kiara's effect is significantly better against single boss fights and the extra energy cost shouldn't matter much in AoE fights with weaker enemies. However it's pretty much impossible to keep it up for extended periods of time. I'd still take Erika over both though.
Ves:
As expected, Ves plays very similarly to Marcus. Her War Cry ability is better than Fiery Counter because it's AoE, costs less energy and anyone should prioritize willpower on her so she has more energy (also to unlock skills). However her damage output is abysmal and using Crush over War Cry against the Kraken can be a death sentence, as one miss is enough to wipe the party. Should anyone prefer her over Marcus? Hard to say. Marcus not only can cover provoking against strong bosses, but with offensive buffs support deals a lot of damage because not only his Fireball Charge has good damage ranges, it also has lower energy costs than most offensive abilities. While fighting the Kraken, Marcus certainly contributed more than 1500 damage, probably closer to 2000 and I was frugal with his attacks because he had to take over provoking when Ves was low on energy. I think that against the majority of fights, Marcus is better. However bringing both isn't a terrible idea. Ves would take over provoking, and Marcus would deal damage while supporting Ves with provoking when she is low on energy. MC with Perfect Organism could take over Myu's role in this setup. However in this case Marcus would become the main damage dealer. (Meaning that he should be given priority with gear)