True. I'd forgotten about that.
So if he can't even do that what can he do to be on par with the apostles? regenerate infinitely? His powers seem kinda underwhelming...
Fuck it, I'm bored and procrastinating other important things I should do.
Top 4 Reasons why the 3rd Apostle is op:
1: Can mimic the form of any and all monsters.
The mc is just dipping his toe into this now, and we've seen it somewhat with deryl's chimeras. But taking it to the level of what a apostle level monster would be able to do, it could very likely copy the forms and physical capabilities of the other apostles bodies. So, however strong the other apostles bodies are, and whatever inherent physical advantages their bodies have, the 3rd could probably do the same.
2: Become different states of matter
We've seen this a little with mc turning into a liquid to move around and escape from traps, but progressing this capability further, imagine then if the 3rd could turn itself into a gas, or even plasma. Assuming this was possible then, the next logical usage of this power for something on the level of the 3rd, turning into a miniature sun. Just change your form of matter into plasma and keep increasing your size and mass until you reach the desired destructive effect you want.
3: Upper limits of increasing your mass?
Continuing on from 3rd being able to increase their mass in reason number 2, what happens if you then keep on cranking up your mass and just keep on going? A black hole is the obvious answer. All the third needs to do is keep increasing their mass until they more or less become a miniature black hole and suck in and destroy anything that even thinks of coming close to it.
4: Invulnerability
The last reason that comes to mind atm, is just an improvement and the logical progression on what Ella showcased in her fight with Nico and Malik. In that fight she locked the state of her form such that it wouldn't change, not from injury or any attack the others threw at her. The drawback in her case was that she was unable to move because that required her letting her form change and not remain static. Well, improving on this ability to the level of an apostle, imagine if Ella or in this case the 3rd could keep their physical form unchanging, while still being able to move or let other changes happen that only they want to happen. The effect in that case would be more or less akin to invulnerability, I would think.
p.s. I think memory has way more potential, but the question was about the 3rd so...
p.p.s. Anther big reason is their likely infinite regeneration, but you already mentioned that one.