If you got lazy and forgot to spirit shell early enough for the AoE, you can get your spirit shell up to the cap with a single PoH cast by combining it with Inner Focus and Archangel. It's pretty broken.
Delay spirit shell a little to line up with some incoming damage, but don't delay it very long. It's a very powerful CD because it scales with your mastery so well. It's so short that it'll be back up soon enough. Use it fairly often.