Give one of your units Killer and Overwhelm.
Card sounds may differ from in-game as they get updated.
Killer - A unit with Killer can make one special attack directly against an enemy unit. This can be done the turn it's played, and can target any unit, regardless of Flying, Aegis, or other special abilities.
Overwhelm - A unit or spell with Overwhelm will deal any excess damage to your opponent on your turn (does not work while blocking).