Well could be few things. First are you having a clean purple phase pull in at 60%? What I mean by this is when you hit 60% you don't have him jump right before. When we were first trying this fight there would be times he would jump right at or close enough to 60% that he would do another jump and the enrage timer would had started while him being in the air, coming down and tank getting aggro and the dot then he would actually start the purple circle/pull in. This really confused us because when we were in final phase at times he would seem to enrage really early while others being fine. To stop this we just always waited for the jump and stopped dps at 62%, have tank1 get aggro and get dot, tank2 taunts while tank1 runs behind wall so he doesn't get pulled in and proceed with final phase. Also make sure your dps starts dpsing right when they are able to after the pull in, during the transfer to wherever you tank kephess during final phase.
Now for the insta dieing part, after tank get punted back make sure he stops all dps and he just runs to wherever you place circles and other tank taunts during the casting of breath. If the tank placing circles is still doing any dps at all this will kill him as kephess will jump/teleport to your tank and 1-shot him using no escape. This took us forever to figure out as I'm a PT tank and was always trying to squeeze out that extra dps we needed while i was placing the circles and could never understand why I was insta dying even though it was way after other tank took aggro and I was already on 2 or 3rd placing of the circles.
One last thing is to have your tanks save their def cooldowns for when they tanking that last 60%. After the first tank gets knocked back and 2nd tank takes over he should use his def cooldown. This should make so he takes very little dmg so the healers at this point should be able to pump out some dps also. By doing this we always beat enrage timers now. Well hopefully this helps and good luck
~V