What exactly does
Sandy's 10-C key have for equipment/powers and abilities? The page doesn't make a distinction. I presume that it doesn't include her
hibernation rage state, but does it include her inventions?
If her 10-C key is just her without inventions, then
RPC-286 wins by gnawing its way into her suit or wriggling into it through her tail hole before crawling up her body and ripping out her jugular.
If Sandy has the
Vortex Machine, then I presume she could win
by sending RPC-286 to the Doodle Dimension. If Sandy has the
Molecular Separator Ray, she'd likely use it against RPC-286 and kill it at least once, but she can't get rid of it permanently, and then RPC-286 could potentially kill her. If Sandy has the
Knockout Ray, she presumably could put RPC-286 to sleep, hijack a nearby car, drive it as far away as possible from Central Park (SBA) within three days, dump it outside the vehicle, and drive back to Central Park and wait a week, making her win by
BFR.
The major variable is whether RPC-286 allowed to kill itself/die and respawn (which isn't specified), and if so, how many times before it counts as a win for Sandy. Since Sandy lacks
Existence Erasure, I don't see a way in which she could permanently kill RPC-286. I'm also presuming RPC-286 is not allowed to use materal from its previous corpses, corpses from nonanomalous squirrels around Central Park, or from humans within Central Park to make equipment, since that would put it in its 9-C key, and that it's aware of this, because otherwise it would, as that's its main combat strategy.