Responses above sound right.
I was wondering, are you bound by 4E rules to allow such an attempt a chance at directly succeeding?
Perhaps a hit from the wizard throws off the warrior's perception somewhat (or even without that playing into it)...
The warrior's first attempt causes severe wrist wounds (and damage). The next causes severe wounds, but higher up on the arm(s). So on and so on without the hand(s) getting cut off. If specifying that an attempt of cutting of hands has a chance at working, why not just specify 'Im going to cut off the wizard's head.' (My apologies if I'm just way off, I'm not familiar with 4E, but I would be surprised if such a 'called shot' were given a legit chance. It's not like he's just saying he will trip or grapple.)
If I were a wizard, I would rather someone slash at my hands/arms than go for a grapple. In the case of grapple, and/or someone grabbing an arm to aid in cutting off a hand (in 3.5 and Pathfinder at least) - Grease cast ahead of time on self gives a bonus vs grapple.