Very Very simple as the online MP component is virtually DEAD.
Go to Online Ranked Match
With your boosting partner instruct him/her not to fight back.
Approach them and press UP on the LEFT THUMBSTICK and continuously press the "A" "B" "Y" or "X" buttons until then get knocked out. When the match is over you will unlock all of these achievements:
Ungroundable
Ungrabbable
Immovable
Untouchable