The biggest problem I have with Mantra is... it's 5%. Obviously if you're absolutely never tanking anything (like Ravana OT), then it's better than Awareness which would be completely useless (don't think the cleave can crit). But in any fight where you're actually tanking a thing for any period of time, Awareness is better, especially in fights where you're only tanking for a short period since you're getting more use out of Awareness.

Also Flash is pretty garbage even for MTing Ravana. The aggro it generates is minimal and unless you're on the verge of losing aggro, the aggro you generated won't matter.