Protect and Shell are your friends, but not Reflect.

Hydraulic Press will do damage equal to your HP, if it doesn't miss (it sometimes does). With Protect, the damage is halved, rounded down. Ultima that it uses to counter is unavoidable, so Shell it up with MDef > 125, and it should only do 6k. And Press is it's version of a normal attack. Don't worry about that one unless it decides to do 99999, because it likes to do that.

Avoid all elemental Magic. All of it.