OK, I've tried quite a few times, and just can't quite seem to kill this thing. I keep dying from the fireballs that seem to be undodgeable. Anyone have any tips on how to kill this thing?

I was playing on genocide, and I kept dying. I tried to cheat and go god mode just so I could win, but I wasn't able t activate it. I ended up hiding behind a rock in the water and shooting him from there. He couldn't hit me back. After all the difficulty I had beofre, it seemed a little anti-climactic.

That's how I ended up killing all the mini bosses; I would just go back through the entrance and long range kill em. Just couldn't find a place to do that on this one.

Just hide behind a rock on the right, near the boss and he won't be able to reach you at all. Takes some time to figure out the spot.

Agreed. Once you figure out you can use the rock as shelter..and even ,if you lean out carefuly, can unload your arsenal on The Ancient One while he is unable to touch you,he becomes as big a wimp as Tchernobog was in the original.