Best cross class abilities for a Warrior?

[Maelwys]

What from what I've seen, tanks tend to die from the crit autoattack after a Death Sentence / Mountain Buster. Not the DS/MB itself.

Yup. In the time between taking a BIG DAMAGE hit and getting healed, you can get hit by an autoattack and/or a random instant-cast ability which takes the last bit of your HP away.

Ideally the healers will be precasting cures before the hit takes place so that you get topped up again pretty much instantly... but unfortunately you'll often not find yourself in "ideal" situations (Lag, Sudden need to dodge or rez someone, Random Derps, RL aggro, etc). That's why "eHP" is considered so good for tanks - you might not technically need all that health and mitigation in order to survive the hardest-hitting abilities in a fight, but the extra buffer gives your healers more time to react. It doesn't all have to come from the tank though - SCH shields and Stoneskin and Virus and Eye-for-an-Eye etc. all help. We lost virus rotations and Bard RoD spam, but the stacking damage debuff from WAR and PLD seems a pretty decent substitute post-patch.

Awareness is no longer bugged, so using it on the offchance of a mistimed Autoattack Crit doesn't do any real harm... but it's still pretty poor in terms of helpfulness. Not sure if I'd want to trade Mantra for it on Bosses (assuming you're willing to drop Internal Release for Mantra) and Haymaker would almost always be more useful on anything that isn't Slow-immune. In fact, even Second Wind (guaranteed light Off-GCD Heal at just the right time) might be better for those "Big hit + Crit Autoattack" situations.