That's because you have to time your attacks with Noctis in order to deal a lot of dmg to Garuda. Doing link strikes speeds up the battle a lot.
As for the OP problem, have you tried going to the monolith opposite to the one that Noctis jumps to? It seems that's the one you have to jump to, also stay behind the monoloth before the AoE goes off. That's how I survived and I also had a lot of trouble with the fight till I tried that.
If you still have problems after that, try posting in the bugs forum.
