Ready one of your units and give it Endurance. It gets +1/+1 this turn for each of its battle skills.
Card sounds may differ from in-game as they get updated.
Endurance - Units with Endurance ready at the start of each players turn, so they can attack and still be ready to block during the enemy turn. Additionally, they are completely immune to enemy stun and exhaust effects.
Battle Skills - The Battle Skills are Aegis, Berserk, Charge, Deadly, Double Damage, Endurance, Flying, Killer, Lifesteal, Overwhelm, Quickdraw, Reckless, Revenge, Unblockable, and Warcry.