SUCCESS
ok new real sstrat - keep the boulder near you at all times, get the guardian as close as possible. when he fires, attack by hitting the top of his head, you can get 4-6 hits in like that.
he will not move, so you are free to keep hitting him and blocking shots.
however, he will prematurely shoot (not fully charging) if you block yourself after hitting him. dont be TOO close to the boulder when he shoots, or you will take up to 1 heart of damage