Like the Green Gnome King, and the Yellow Gnome King. Fighting the boss will be tough, so first, make a great distance between you and the Gnome King, and take out the smaller units. If you are at full health, leave the shards until you absolutely need them. Once you have taken out the smaller Gnomes, work on attacking the Gnome King; use your ability when attacking him. Take cover when your ability is recharging. Repeat this process until the Gnome King is defeated.