You can control a pet by holding down the alt key and using the right mouse button.
Sometimes the best defense is more offense; use Mordekaiser's abilities to charge up his shield.
Remember that Children of the Grave can be used defensively because of its lifesteal component.
Mordekaiser's abilities charge up his shield, making him tougher to kill over time. Try focusing fire on Mordekaiser to burst him down.
Keep your distance and don't stand near creep waves since he'll be aiming Siphon of Destruction at them.
Watch out for the Children of the Grave and Ignite combo. If you're low enough, this will kill you and give his team a powerful ghost.