You'll be able to do this after claiming the Battle Tower. You WILL get this achievement, so you really do not need to worry about it.
Some tips on fighting the boss though:
Take out all the Abmoninations, they will be a pain in the ass. You can do this easily with Exploding Arrow (yep, if you've read my other solutions, I'm stilling pitching this cuz it works).
Have Sussan either Healing or Attacking, her creatures didn't do much when I choose that option. Hopefully, the boss will focus on her, that makes things easier.
You should focus on Razekal, killing him kills the boss as well.