Final boss got replaced with Putrid Worm.

[Reformatio]

I captured the final boss; now the whole encounter just let's me fight a Putrid Worm. I can neither capture the Cosmic Gigalith nor finish the game on Epic.

As far as I am aware both enemies do not appear on boss rush.
Is there any solution? I want all the achievements ^^

[Sovereign in Exile]

There is no way to fix this besides starting a new game, or reloading an old save file.
But you can't get epic achievements on new game plus anyways.