Soul Strike (MP15-EN247) - 2015 Mega-Tins Mega Pack 1st Edition

If your LP is 4000 or less, when an attack is declared involving a monster you control and an opponent's monster: Pay half your LP, then target 1 face-up monster you control; it gains ATK equal to the deficit between your current LP and 4000. You can only activate 1 "Soul Strike" per turn.