Pull grond into the moving helicopter blades inbetween the two runways in area 51 to knockback grond and deal a very large amount of damage to him. I suggest clearing the area before grond shows up in it so you can pull him into it for the two times he "jumps" into area 51.
Note: IF you have broomsweeper I've heard you can get grond "stuck" in the blades. Instantly killing him.
Choose to use it however. Me and my group did that off the start before he left area 51 and had him at 50% before we even really begun to fight him
And heres a jump map of where grond shows up: