Helio Breaker guessed spiritomb up above, that's not it? It looks like they ninja'd you so just making sure you saw it.
The only other thing I can think of is Spiritomb
I am no speedster, but I don't have to be only 35 base speed
I'll probably hit you before you hit me Sucker punch and shadow sneak
Even if I don't, you'll still see my worst Foul play and pain split
Pick your Poison, or Bug, or Fire, whichever comes first Toxic, Will-o-wisp and infestation?
I can shield myself from your bouts Protect/substitute
And watch as you slowly die out
You'll see from the grin on my face Has an evil grin
Slow and deadly wins this race
I rotate in place, awaiting your move It's sprite slowly spins but the keystone doesn't
Only to rock back and launch foward This line might refer to the keystone itself as being a rock
Right before you do
Forgive me if I take too much time killing you
You sweepers have short memory
But my laugh, oh! My laugh!
...It sounds like Hell's machinery It's cry actually sounds creepy and almost robotic
That's the only other thing I can think of
Oh! I totally missed it!
YES! This is it! I'm so sorry for missing this. I should've checked for ninjas.
I must say, though, the rock back portion is a reference to its physical attack animation, which is itself a follow up of the second line of the first clue, Shadow Sneak.
Well, you got it! You're up next, Helio!