Ambush to Kill or be Killed

Target creature you control gets +1/+1 until end
of turn. Then that creature fights target creature
you don't control. Whenever the creature you
don't control dies this turn, put a +1/+1 counter
on the creature you control.
"...You have five seconds to state your purpose."
"Man, what they say about you is true! there's
no sneaking up on you!"
"Your time is up-"
—Papyrus to an ambusher