I find the heals from Ice Yendor annoying to kill the giants through, much better to keep them separated if possible.
How much stuff do you usually end up fighting at once in room 3? Rooms 1/2 are 8 giants and Yendor at the same time if you face it head on, room 3 should at most be 8 giants with no Yendor if you move down.
If you can keep room 3 to just one group at a time, I think you'll find it to be easier than rooms 1 and 2 for sure. Blast two of the four giants, and then the other two can be killed with ice knockback regardless of what you have.