Just to clear this up, I've completed that challenge and it must be exactly 8 hits. Here is what I used: Jump punch, 2,1,3 Dash 2,1,3, flying kick. Or if they're in a corner, replace flying kick with an uppercut. Easy money.
Weird, i tried it today so i did 2,1,3,2,1,3,1,1,1 result: 9 hits, 0% damage, no win.
Okay so maybe it had to be exactly 8 hits, i tried the same combo without the last hit result: 8 hits, 0% damage, no win.
Anyone else having this problem? Or is my game just bugged or something?