What a fucking shitty arc, stop it already.
This arc has nothing,
- we don't learn anything about the characters (they are the same as in the "present time")
- we don't learn anything about his capacity, the stakes are null
- and the tactical aspect of the battle is as dumb as this manga (omg, a monster that absorb attacks, and regenerate itself. It's so hard what should we do ? We will use a card that I happen to have and say "can't be absorbed and prevent regeneration")
The only thing good is the drawings but it's wasted in this arc.
It actually makes sense, boss is constantly evolving around (elemental) magic, not any other attacks.
That is not unusual for RPG bosses.
And as someone in here also mentioned, it wasn't even venom. It was physical slime(which are famous in many stories to digest anything) mold, which the boss has no resistance nor learning ability against.
You don't have to like it, but no need to make it worse than it is... It's already cheese shounen enough.