Strax, Sontaran Nurse

Vigilance, trample
Grenades! — {2}, {T}, Sacrifice an artifact: Choose a player at random. When you do, Strax fights another target creature that player controls.
Glory of Battle — Whenever Strax deals damage to a creature, put a +1/+1 counter on Strax.

Doctor Who

If Strax is dealt damage equal to its toughness while fighting, it will die before the +1/+1 counter from its last ability can save it.

The player is chosen at random, but Strax's controller gets to choose which of their creatures it fights.