So i've beaten this game twice (only with druid, creations seems way to confusing and havent tried female playthrough yet), on the third challenge and it always seem like the limiting factor is the strength of my monsters. in the beginning i picked fusion but now im leaning more towards breeding. on a short game length (30+ days) is it feasible to get a monster to level 20 with +10 in genetics? Honestly it seems like healingflames or eruption with a phoenix is the most op abilities but they both scale off of mg, so i try to fuse or breed characters with high magic genetics but even when i get some monsters to like 6+ mg it seems when they level they never gain mg in their actual stats. a wiki or guide outside of the knowledge they provide in game at the library would be a huge help.
Yes, it's possible to get some lvl 20 with 10 genes in a short time. I've gotten there at around day 24-26, more than enough to actually level up afterwards. It's become even easier than before if you pick the "breeding focus" in the farm, or the starting "farmer" profession (boosts number of rolls for stats too, huuuuuge help).
My personal favourite is to go in a portal with capture nets and get 2 baby dragons from a mountain event (you can't capture the adult in the egg ambush), then breed them together. Doable on day three to five usually. The lava eggs mature very quickly, so if you're lucky with the stats rolls and have some crystals to spare for stability (for dragons i only aim for level, health, strength; magic is optional but good to have).
To accelerate the process, you can farm spirits on the side to get a big early max-level boost by fusion. If your summoner has high base magic power, the spirits will also have more base genes.
Scaling is geometrical in value by adding valuable traits to your minions (easiest on Evolution). A dragon with 6 health gene, 7 strength for example, will get 20-22 STR at lvl 19-20, more if you rank him up in the arena. Add "strong" trait, goes to 25-27 (+25%). Get "potential" from birth on top and it's now 32... get the "hatcher" start bonus and he'll have the loyal trait buff, and now it's 36 str, all at the same level.
Note that if you really farm the dragonwelps to genLVL 20, when you evolve them you'll get lvl 30 max dragons! (only way to unlock a particular trait as starter).
Another factor to consider is the species you're trying to give each role to. Each species has different scaling for every stat, which is added to the leveling type (support, dex, magic...) that tweak the level-up value. For example, getting HP gene 10 on a species that has HP scaling at x0.5 is not bad, but you're better off with it on someone else that has a HP scaling of 1.7 for example. Not every monster can be a mage, not every monster can be a tank or a fighter.
The last is the creature's size, with a stat multiplier ranging from 0.5 to 2.0. Just keep in mind that portals are restricted to certain sizes, most can take large creatures but above that is more rare (see portal info before entering).
Finally, all that information is easily accessible in the stat page of your units. Hover your cursor over their lvl and current xp and you'll see the species scaling for each stat. Hover your cursor over the LvType and you'll see the level-up bonus baseline for the class. Hover over their size and you'll see the stat multiplier and food cost.
As a player, i haven't tried to dig in the game files to know the exact impact of genes, and the level-up scaling formula, so i'm not sure how much genes matter. i just know the impact is noticeable and haven't tried to measure it comparatively with a 1.0 scaling species somewhere.
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Now about the different characters...
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Evolution is by far the most powerful on her own, the ability to transform is both versatile and powerful. It's also much easier to perform the rituals, because you can change her sex to whatever you want on the fly. On my latest run i went for an early lust transformation (20+ per seduce around lvl 10), then changed to mage type. Now i'm lvl 25 with Evolution, have NOT maxed either transformations or rituals, and i've got... 86 magic power. Yup. 45 base+level mult+traits+items. The ability to copy/paste traits from one monster to another is completely OP, you can make an army of optimized minions very quickly. Evolution's power curve is fast overall, and slower towards the end when you're farming the last transformations, but her personal power is unmatched. End-game in god-form, with the lightning shower spell and a mage built, you can literally nuke 90+ damage on the whole enemy team every turn, with Evo alone.
Creation is harder to play, because her genes actually matter. You have to pay for it with your starter points pre-game, you can't boost them during the game whatsoever, and you're gonna need to breed/corrupt some slime monsters early to change your LvType from "average" to "mage/endurance/support". She's much weaker as a unit throughout the whole game. Her main advantage lies in being able to create the monsters you want or need if you've done a ritual with them once, and to choose their spells and passives, sex/size etc... The start is slow, you're better off earning money in the arena and brothel to get some base species going, after which you can breed every combination easily on your own. The power curve is much slower at the start, but by endgame all your summons will be gene-maxed...except that if you've been breeding stuff on the side, your summons will have the stats but not the traits so they'll be strong but farm from OP.
The slime talents tree can bring some personal power and global utility (like removing the restriction on partner sex for rituals), but you won't reach a godlike level of power with Creation herself. Slime are a bit like Shadow monsters, in that the more you have in the team the better, but if you only use one it's gonna have a hard time.
Creation's concept is cool, and her personal quests have much more grit and intensity than the others, but her powers fall a bit short of the fantasy, tbh. Her mechanics are eclipsed by the base game mechanics (creating/corrupting vs breeding/fusing mostly), the late-game power fantasy isn't really there.
Druid is the classic-RPG character, nothing much to say about his leveling. His personal power won't skyrocket, but it's very easy for him to get powerful seedlings very early. Evolve a seedling to a seedling-dragon asap so you have an all-purpose large flying mount to reduce travel time, it's powerful even at lvl 5 with trash genes. He's got some interesting gimmicks with grass surfaces, and later in the game with blood fairies if you go that route (you monster), but in the end you'll still rely on breeding to get a good end-game team. Personal early strat is to train with Aila in the evenings until i get the gene bonus to seedlings in the talents trees.
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Conclusion : holy fuck i wrote a whole lot, didn't i.
EDIT : each character has unique endings, with unique consequences for future playthroughs. You might wanna look into that.