By the way, just some tips for beginners:
- Play the tutorial first, many of your questions can be answered there.
- Commonly, all squads must have at least one Guard (Shield Icon) and Healer (Skill: Division Heal, Demon Medic, Equitable Heal)
- Recruit competent DPS as soon as you can. Horse Musketeer and Dragon Knight are plenty strong in the first few stages since they have Sap Flying and Slay Flying. Very Useful when you have to contend against Milia's dragon knight squad. Incubus is quite powerful as well since it has double Slay Woman and Sap woman.
- Invest in buildings as soon as you can, it will unlock more buildings and even a new floor.
- Don't underestimate traps like Beast Graveyard and Jail. They can stop and weaken enemies by a lot.
- Build your squad around one theme. For example, you can build Man squad with Rein as its leader, or you can build a Night squad with Ether Wisp as its leader.
- Don't forget to equip items.
- Change your tactics. All of your squad has [Attack] tactics by default, surprisingly, for this game, [Defense] tactics is way more useful. My divisions got wiped out more often when I haven't changed the tactics to [Defense]
- Forward guard is the ultimate guard skill. If you don't know, the skill Range Attack and Flank Attack make your enemy attack bypass the Guard unit. The only way to reliably guard against those is using Forward Guard. The normal tactic is by placing either unit with the lowest level or the lowest defense in front of Guard with Forward Guard.
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Those are good tips for beginners starting out.
For ultimate late game set ups (for higher difficulty berserk), you need to set up evasion units to dodge dimension slash. That means an evasion decoy, or using defenders that can get high evasion along with the usual tanking skills. Evasion decoy teams will require flank null. Evasion decoy means having the lowest HP x Def unit in slot 2 with 100+ evasion so ramged attacks and dimension slash targets the decoy, and having the 2nd squishiest unit in slot 6 guarded by the defender on slot 1 with forward guard or target miss.
Another significant mechanic is that barrier skills are multiplicative on different units but additive on a single unit. This means reaching 100 barrier on a single unit gives you immunity, but having it across multiple different units won't.
So a perfectly set up division consists of cross+all null, flank null, evasion decoy/tank, a tank unit with 100 slayer def and 90 hardy physique, 100 self destruct and 100 spell barrier, absolute cure, and enough healing. If possible, resist ailment or dauntless on the tank to block charmed. One sweeper with all/cross attack. And one heavy hitter that attacks in melee with high crit rate to bypass parry and enough defenses (counter res, slayer def, hardy) to survive counter attacks. After all that, it doesn't hurt to get some ambush alert to prevent massive shifts in formation (4+ shifts) that messes up the entire set up.
That's all super end game stuff though.