9th Dawn III

Once a creature reaches level 90 you can use a morph stone on it. There's a few tiers of morph stones and if you care about the creature's stats, use the legendary version, which can be bought in Coulsteel, likely the last town you run into unless you skip Holdune. When you morph a creature it's basically like a prestige system. They delevel to 1, lose some of their stats, but as you level up it'll become far more powerful than it was at level 90. Note that you can go to level 100 for better stats, but it won't mean much if you don't care about the creature and only want the achievement. Note there is an achievement for a creature reaching 100, so you'll want to wait that long at least once.