Aven Tactician 

Creature — Bird Soldier
Flying When this creature enters, bolster 1. (Choose a creature with the least toughness among creatures you control and put a +1/+1 counter on it.)
Legality
standardNOT LEGALpioneerLEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL