On the exact same flash point, had that same issue.
So roughly was LoSing (healing to full doesnt work BW) popping force barrier when I was about to die, healer took some heat, when he was under 50%, I was taking the barrier back down and we ping ponged the aggro between ourselves.
Interesting "how to survive when BW doesn't want you to" FP.
Gehe, I know exactly what you mean, had quite some red reaper fps myself on my slinger where I ended up tanking.
But correct me if I'm wrong, in PvE the barrier works as an instant aggrodump. Meaning, pop it and all mobs go for someone else. Release the barrier and they are still going for others. I never tried it out as sage dps since as a healer the mobs will glue on you again because of the healingaggro.
Last boss was fairly uneventful compared to the rest: Tank and Marauder ignore kill order and attack commander first. Sorc DPS both of the two big Raks first and helps the other guys kill off the commander.
I gave the Sorc the speeder afterwards for putting up with all of that.
Nice and weird story
However dont judge to fast on the killorder thingy. They only ignored the killorder which you are used at. The tank needs to hold aggro on the commander in all possible killorders, after that you can decide to burn the commander so that the 2 others become tankable or burn the beasty who pins you.