The Mimic's level scales with Celica, so even having companions with isn't advantageous unless their levels are close (everyone should be able to hit the 80 cap). It is also ridiculously fast, so having a Speed of 50+ helps tremendously.
EDIT: Letting Nel power level you (and potentially two party members) also has the advantage of never encountering a Mimic.
imo having Nel encounter the mimic would at least semi balance it :/ she literally regenerates both health and mana mid fight. She also seems to have a much higher chance of just breaking out of being grabbed, and has a speed bonus. Because hot damn does Nel get a lot of bonus stuff we just don't get >_> seriously, the number of times she auto breaks out and attacks same turn is much higher than I get. And I run relatively high will and even wear that one item for the increased struggle chance. Regardless, if the mimic scales to Celica's level for damage, fine. But it shouldn't be able to just lock you into a death combo of attack then grab =_= let alone sometimes just choosing to go 4 attacks before you get 1
