There’s only a few enemy types who can can one shot you though, like snipers, which is exactly the same as Halo on Legendary, and in Halo you don’t even know if they’re aiming at you. Other enemies, like pouncer spikes, they’re effectively minibosses so high damage is to be expected and was exactly the same in GoW4, except you didn’t have Jack then. The other enemies, you do have time to get into cover once they start firing, or to retreat if they have a shotgun and you see/hear them coming. In tougher situations you can get through by using Jack properly, including against snipers and shotgun enemies.
Which part are you finding difficult? The only part I found frustrating was a fight against a carrier and flock at the same time, but that was partially my own fault for not managing my ammo properly and also the game despawning dropped weapons after reloading. There was also an autoscroller section that was a bit frustrating, but again that was my fault for not collecting ammo. The amount of damage the enemies do is normal for Insane and you have to change your tactics . You wouldn’t play Halo on Legendary in the same way you’d play on other difficulties, even Heroic, because like in Gears you’d get killed very quickly.