It's somewhat disappointing that this got 4 pages long before someone mentioned that marowak gets priority stab earthquake (when using prankster nature power) coming off 568 attack without life orb recoil or being locked with a choice item.
Slaking won't kill with Extreme Speed, but if you're opponent packs Fake out you're in trouble. There's also the issue of you dealing a measly 40.47 - 47.81% to Slaking in retaliation. Heck even Hi Jump Kick does 70.23 - 82.93%. In a metagame where the revenge killers have attack stats over 1000, Marowak needs to be careful. That being said you could turn Marowak into an amazing Baton Pass counter with some combination of Magic Coat/Nature Power/Taunt/Heart Swap/Copycat/Whirlwind.