How much hp do i need to survive zak?

Zakum hits 3.5K at most. That means you should have 700 hp to survive. You said you have close to 1000 which is good enough with magic guard. One thing though, don't hug the wall. Usually, when it does it's fire attack, the area warning shows up exactly on the wall. That means that if you lag during 1/1 and you're on the wall, you're screwed.
You want to stand somewhere between the wall and the end of the platform. (above you).