Ephemeris
Member
but in my case I just don't get any prompts other than the ones above his head
That's because you're hitting it too quickly at that point.
See if you hit the counter button at the start of his barrage, before you get the official prompt, you will get pummeled.
That unblockable counter is actually him starting his animation, you accidentally/purposely hitting counter way before the game prompts you to and him just finishing up the damage.
Look at it this way. Slade has no unblockable counters
Until then, enjoy his awesome trash talk and game-over screens : D
(I'm being sincere. I really dug those)