Just play them. They're immortal!
No, seriously. In my experience the Guardian Angel is oneof the most durable creatures ever.
They come with Aegis 1, 1 Armor, a Defense and heal 2 each time they choose the guard action. Unless they get focused down they give your oponents a hard time. Oh, and they're flying as well, so most creatures can't even attack them.
Most of the time, a killed Guardian Angel gives you a huge mana and action advantage. Just throw out another one.