when the big Rak is deemed the first to die the tank often seems to forget the commander is tauntable.
If this is a problem, it's because the tanks you're running with a bad. Any tank should be able to maintain aggro on the commander Rak purely with tangential AoE (Guardian Force Sweep and Guardian Slash, VG Pulse Cannon/Mortar Volley/HiB cleaves, Shadow Slow Time and Force Breach). On all 3 of my tanks, I completely ignore the other bosses and almost never have a problem keeping aggro on the commander purely by virtual of tangential AoE providing all I would ever need. I think the only time I actually *do* have some issue is when there is a well geared Gunslinger that drops Flyby right on top of them and, even then, the time it takes to land the hits generally gives me more than enough time to build up enough threat to not have it stolen.