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sjmcc13 @
Riri
If it's a traditional Mimic [that is, from settings where they're high-level creatures, rather than trash mobs ala D&D-style settings], it has overwhelming defenses against non-magical attacks, strong defenses against basic magic, and strong, penetrating attacks that can perhaps even destroy even magical equipment. As startling as it is that he'd survive an attack from such a creature, "erosion/decay/negative-energy magic being its weakness" isn't too far-fetched a consideration. It being strong enough as a level 1 spell to insta-kill the Mimic? Yeah, that's a bit more iffy. The whole thing feels forced, but it still feels fairly reasonable on the broad strokes. So it's about the same as most any manga of this style.
That said, the one thing we CAN acknowledge, is that mimics aren't fast (as sjmcss13 touched on). The entire reason mimics camoflague themselves and don't typically initiate attacks is *because* they're slow, despite their strength. Their usual strategy is to surprise attack, and then move to cut off the escape route, using its superior defensive stats and its ability to transform out appendages such as tentacles to its advantage in dominating the small room it has taken charge of. Again, it's hard to explain why the mimic buggered up its initial attack pattern so badly [maybe it wanted a sample bite, and ended up not liking the taste of the MC's arm?] but the (MC escaping a level 500 creature) bit is actually one of the few fully justifiable considerations in the encounter.
That said, I've decided to interpret the matter with the mimic being comparable to a slime. Changeable, overpowering, easily regenerated outer body, fragile inner core. So I'm interpreting the matter as [amazing regen and adaptability] countered by [MC destroying core with a lucky shot]. Seems like the best way to explain the insta-kill.
That or the mimic has been spending all its level-up bonuses on gourmet skills.
Also, I'm pretty sure the mimic was completely wasted at the time of the encounter. I mean, look at it stumbling to form coherent words, and stumbling about. Clearly drunk.