also make sure that you're hitting the enrage and not the 15% burn phase. you said you get him to about 13% and then he enrages so it might just be the burn phase. the first couple times we killed him we started falling quick during this phase and barely downed the boss. (the first time we actually wiped on the adds that spawn but managed to kill the boss) when he hit's 15% make sure you're dpsing the boss and not the adds. have your off tank (the one that happens to not be on the boss) taunt the adds and pop his def cooldowns to give your dps more time and make sure your healers still have force/energy at the end of the fight for this. so in short use a timer and if it's not the hard enrage just make sure you're handling the burn phase properly.
thanks for your reply.
At 15% we are still working through the last bit of the third male and adds. The WH turns red and hard enrages.
This is confirmed by the buff in red "enrage 200% damage increase.
Again the loss is the dps somehow, but where? It's not gear.
Fine tuning and advanced mechanics me thinks.