GMS:CrimsonWood keep 2,Magatia coming soon

That would make it not a challenging battle, but a tedious, annoying, and money wasting battle.
*attack*
*invincible frame*
*boss moves*
*take damage*
*pot and move*
*attack more*
*invincible*
*boss moves*
*take damage*
*pot and move*

And a boss with those kind of invincible frames would only be use for 1 person, considering if you have like 12 attackers, only one will get their hit in while the others waste MP and take damage.

Not random annoying invincible frames. Well placed ones. I'm fairly certain it could be done.

It won't be, but that's another matter.