Airborne Armory

Strategy
Airborne Armory allows Flying Heldim to exist. If you manage to fill your board with Flying units, this can become a massive threat that's hard to get rid of. Note that it also buffs itself, making it a 1/4 and keeping it outside of Fireball range.

Airborne Armory also works well with The Breathing Nest, which can summon a whole lot of small flying units. Airborne Armory makes those units not only strong, but also prevents 1 damage AoE from destroying them.