Well, do mind one thing. I have the fight sorta planned out, and Death itself saw that Lucas got involved in it - meaning Lucas will be at least instrumental in finding the way to - if not kill the creature outright - defeat it.
In fact, I do have her secret weapon planned. :)
I don't condemn Mary Sues, as, well, I know for a fact that I sometimes run the risk of turning Lucas into exactly that (I hope that's not HAPPENING though ;) ).