If possible, target off the raid assist target. Many times in a raid, the boss has 4 or 5 rows of debuts that are horribly slow to redraw on your screen. So you will be very laggy when trying to build gambits. If you target through someone else, the redraw is much faster and you can get off gambits faster. I've tried this many times and its a very noticeable difference.
I have the same issue though, I can't keep all defensive buffs up along with all tiers of HoTs at once. I focus on defensive buffs and a T3 and T4 HoT. The extra pulses from traits and the hytbold 2-piece bonus make that much easier. I throw in a double Agression in between each rotation. Your HoTs will add single target threat during the process as well.
Gladvo R6 Burg, Heartscar R6 RK, Morrall, Baldisbeautiful, Makeyoubleed