This build may not be possible outside of CoA server.  It achieves a little over 25 seconds of invulnerability per Avatar use with around 4400 max spirit, and can activate Avatar again every 15-20 seconds, leaving breathing room in case you get CCed, have enemies spread out, or otherwise can't constantly attack.

Nunchuck Bulldoze is in the build to build spirit faster.  It is easier to refresh your avatar with that power than without.  If you're fighting a target dummy or boss you know you can stick to, replace Nunchucks with Tripwire Bolas and switch to the talent that adds a DoT to it.  When constantly attacking, this build still has breathing room without Nunchucks.

In combat and with 1 HP, I was seeing about 70% critical chance and 40% brutal chance.  This was tested with the indicated IP allocation and gear.

Unfortunately for Moon Knight, there is a bug with the server and client synchronization.  Sometimes, it is possible to activate Avatar before the server tells the client you have been CCed, and your client thinks you have no spirit because you activated it, but it didn't activate on the server side because you were CCed, so you are stuck with 0 spirit on your side and a game client that doesn't update your spirit until you are dead from the bug.  It probably won't get fixed due to the difficulty of patching the game client.