Boros Challenger 

Creature — Human Soldier
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
: This creature gets +1/+1 until end of turn.
Legality
standardNOT LEGALpioneerLEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL