Depend of the pokemon you attacked.
Could have been a high def one
It was a team Aqua grunts mon so a shadow mon. but my point for the high-def mon is that after the sucker punch put it to little less than half they healed it to full and shadow claw took it from full to 0. this is after 4 dragon dances that just doesn't make much sense to me. and Like I said both the sucker punch and the shadow claw were not super effective or ineffective they were neutral. This is why I am so confused as to why shadow claw (even Stab) did so much more. a 3x attack sucker punch should have OHKO'd.Jeez so you had 3x attack shadow mon are pro;ably buffed to be equal with mon 20 lvls higher or something not sure i also dont know how to get a grim skull nineties so can't say anything about stats my best guess dark type moves were not super effective either way shadow are busted and sucker punch does if the other mon is attacking if I remember it's description right