Benjamin Olson
Hero
Eh...I definitely wouldn't waste a PWK on a Druid's wildshape form. I'd probably keep leading them in the sky while casting something like a web, which restrains them and therefore reduces their speed to 0, having them take the roughly 15-20d6 (being optimal). I'd probably then either cast PWK or a high-level damaging spell so for the overflow damage on the next turn. Maybe a heightened Finger of Death?
It's actually pretty easy to grab every single 9th-level spell as a sorcerer if you set out for it. You're probably keeping only the all-star 1st-level spells in your spells-known. Same as the other spells up to 4th or 5th level, so you'll probably won't even need 50% of your spells known for this specific fight. I don't think it's too wise to grab PWK, though, unless you have a specific strategy for it. Then again, if you can get initiative first, a quicken firebolt/PWK could kill a wizard with a +1 con mod.
Waste Power Word Kill on a wild shape? They are DEAD and you have won. I'm pretty sure that is the point of the exercise.