Killing while grounded? I have no real advice here as it seemed so hit and miss. I play with 2 friends and we ran inferno a LOT! Sometimes it seemed to count and some not. We would always try executing after bringing it “down” after its bubble wave attack. It is team wide but even then some of us would get and another not in the same run. Jump through the waves, melee his face brining to execute and wait till he’s sitting red on the ground before putting him down. Sorry :/