Grendel is a easy fight it just takes patience. After you hit him a few times, he will start to spew these annoying little flying bugs (about 6 of them) shoot them or they will swarm and attach onto you.

Shooting them will also charge up your special so that is a good idea. After you get his chest to open it is all about lobbing your secondary fire into his chest. This will just take a long time.

The third phase of the battle is Grendel crouching down and rolling out of his way while still firing at him.

Pretty simple fight actually.