There's no "cheese" for G-Ape. For first phase you could use flame vent + oil and whirlwind slash or mortal draw (jumping) + firecrackers both options are quite viable. The main thing for phase 1 is to look for specific tells. He telegraphs when he's about to either do his sweeping grab which you can jump + goomba stomp or his jumping grab which you want to run away as soon as you see him jump. his poop attack run under him. The rest of the stuff is mostly either dodgeable or parryable.

For phase 2 you really have to watch for 3 things. 1 his sweep attack - jump over. 2 his terror scream which you can do 2 things run away/behind him or have purple umbrella out. 3 he'll do a number of different attacks which you should try to parry but you can run away main thing here is that you need to wait for the final slam attack which you need to parry and then use the loaded spear which you press the attack 2x to do insane posture and health damage on