I'm sure this will work for other classes, and it sort of piggybacks on the door bug alot of people are posting, I've searched and if this is a double, my apologies.
Anyways I have a severely undergeared prot pally (28k health JUST barely def cap, no avoidance ect.) and I attempted to do thekal the regular way... with my gear, well.. the boss and his buddies couldnt kill me... but I could never get them below 50% because of the heals..
So i decided to give the door bug explained in many threads here a shot, the problem is, alot of times if you re-aggro too quickly he no longer resets on the threshold, and you have to run out of the instance.
So I gave it one final short the regular way using hoj/arcane torrent to interupt heals, and after 25 mins was bored and tried to run out..
That's when i learned something.. when I was attempting this, I managed to kill both the adds at one point or another, but not thekal himself... when I gave up and ran out the door... thekal followed me, his adds reset like they normally would have if you ran them to the threshold.
So far I'm 4 weeks using this strat... it's alot slower then bugging via the door but if you get it to where they dont reset when you cross the threshold.. run back in, kill both adds, and then when they rez run back across teh threshold and they will reset and run back, thekal will not, blam phase 3.
I don't know if this will help anybody out there, as it would only really affect undergeared people who cant aoe the mobs down, or cant get the door thing to work without a thousand attempts, but since the instance is being removed I've been trying to get it done on all of my undergeared alts, so I figured I would share.