There are a handful of bugs in almost every phase of the fight, some are easy to spot, while others are little more of a ninja. i would suggest you look through these forums and the CS ones and see if any of the bugs might be causing the additional damage to the tanks.
My guild is also having issues on him, and we have wiped to a few bugs and the handful of times we got into the final phase and had the tanks rolling and dropping their circles, we did see crazy 20+k spirts which on their own aren't so bad, but when followed by 12k back to back hits, we have had issues. We heal with two sorcs. In all honesty if the fight was a lot less buggy and we could just practice healing and tanking more, I am sure we could kill this guy with little issue now. We have been on him for two nights, so about 8 hours, and the bug wipes followed by the long run back def makes the fight less fun. Beyond that, the fight is actually the first one I find real fun to heal.
So yeah not much advise other then check your logs for any odd hits, like abilities hitting the tank when he doesn't have agro, which is one we have seen, and just oddly huge near 1 shotting abilities when they shouldn't hit that hard.
Fight mechanics =/= bugs.
They fixed the bugs that were in the encounter: gift of the masters, pulsar droid health.
On topic: in the purple shield phase, it is a hardmode mechanic that the time between waves is nearly halved. There is a strat that allows you to take your time with the waves even with 0 AOE. I'll leave you to figure that one out.
Hint: the shield has a finite size no?
Make sure you know what kephess' abilities.
Pre 60%: A dot that can be healed through.
A jump that deals damage upon landing and increasing his next swing by 100%
Post 60%: breath of the masters: taunt time
Savage arcing slash: frontal cone knockback, applies damage vulnerability debuff
Otherwise if that's not your issue, it's just the fact that he just hits plain hard. (I'm a healer I have to heal this damage) just tell your healers to go all out.
Good luck. Hope this helps.
P.S @#fraeblood really interested in whatever bugs you say you're finding. Because i can account each of your "bugs" to a flaw in your strat. Care to show me a vid?