HT will always be a pain without the right comp and right gear/level on your ships and pilots.
Buff immunity is awesome against him not only because it allows yo to dispel his taunt, but because it also prevents him from gaining protection up, which makes him easier to kill.
But remember, as other already said, that HT's basic cleanses all his debuff. If you apply Buff immunity just before he gets a turn (or just before he gets called to assist), he will cleanse himself and the Buff immunity would be wasted. The timing can be hard some times if you don't have the right set up, and for Rebels, cleansing the Breach out of your ships will most likely be easier, specially that if there's no other Breach applying ship, so once HT uses his special you will have some time before he can apply breach again. Keep in mind tho that while he's not taunting, HT gets TM when you hit other enemies, and he has higher tenacity. For Rebels, what I used to do was using the Y-Wing's special while HT was taunting (less tenacity), get the Prot disruption on it then burst him to take him out before he can dispel the Protection Disruption with his basic, as he gets extremely squishy without protection and protection up.