Err, you need to change gear if Grell is one-hit killing you. I've killed him at level 13 with ease on every playthrough. That or you're playing online and there's 6 people fighting him or something.

it was a party of 3 but still i shouldnt have gotten 1 hit koed. I feel like the cannon arm doesnt hit just once. I think it will hit you multiple times over the course of a second because of the knockback and the cannon not disappearing before it reaches its destination so the hit detection might read that in as multiple hits in almost an instant.

stay the hell away from his mass-damage area when he casts it. Simple as that and keep running. I've killed him quite easy aswell.

I know hes easy and all that its just, im trying to find a way that if i just so happen to get hit then it wont automatically become a 1 hit ko situation. I mean If I was melee it prolly wouldnt be much of an issue but with ranged equips and trying to run away when theirs mobs all over the place, more then likely (becuase the enemy atk box is so big when you click) you will target an enemy and atk standing still instead of running away. So to avoid that, I wanted to try and find a way to not get 1 hit ko'ed >__>

I mean all my level 20's+ died that way, I try and run away from grells skill and poof instead of running they atk by accident and die. And because its HC i need to start aaaaallll over again. Its frustrating to say the least.......

I've really never had him come close to one-hit killing me. Even on hardcore Elite. I think you're most likely gearing wrong. On hardcore your gear needs to be focused on survivability, not offense.