Not killing Massey first will result in a lot of reflected damage on your group. This can be a detriment to groups that are within that "squeezing" distance of barely being able to down this encounter.
My group kills Massey first, then the commander, and then finally the medic. The medic's heals CAN be interrupted so you can delay the healing he puts out somewhat on massey. The key here is burning Massey as fast as possible. Once he's down burning the commander is a joke. The medic usually enrages for us, but I tank him with my Rakata Shielding relic along with Saber Ward and Invincible, I've lived through his enrage with my cooldown abilities alone.
If your tank is burning his cooldowns just normally tanking this encounter before you get to the medic, then you might want to do False Emperor to get some better gear (namely the chest piece) first.
FYI. My gear in this encounter is Columi Helm, PvP Champion Chest, PvP Belt, Columi Pants, Energized Boots. Other equipment is a bunch of PvP earpieces/implants, Matrix Cube, Artifice Crafted Relic. I'm really not in that great of equipment and the encounter is a joke.
But then again, the trick to beating enrage timers is not your tank but solid DPS...