[This strat has been OUTDATED.]
This is what we learned going in with: 13 people in greens, blues, and Naxx gear, and no clue:
You can walk straight to him. I think. Just kill some trash on the way.
The four adds attack one by one. Then a bunch of weapons attack. Have a basic kill order, nothing fancy, they fall fast because you're level 80.
When the weapons die, you must pick them up and equip them. Everyone can get one or more than one. I suggest, if things are going well, do it before you kill the last weapon.
- Tanks should get the [Phaseshift Bulwark] shield. This has a Use to withstand Kael'thas's pyroblast.
- At least 2 Melee should get the [Infinity Blade] dagger. You stab your friends with it and remove Mind Control from them.
- At least 2 Players should get the [Staff of Disintegration]. This has an area-of-effect Use to make everyone in 30 yards immune to stuns.
While you're equipping the legendary weapons, you only have a few seconds before the 4 adds rez in the spot they died. They are tougher this round and your dps needs to be good enough to kill them before Kael'thas joins the fray.
Once he does, if you're still struggling to get them down, you might as well just reset the fight. You need more dps.
First, you will almost constantly be stunned and disoriented. The staff-bearers are vital for this part.
Second, you will almost constantly be mind controlled. The people on daggers are vital for removing it.
Third, Kael will bubble and pyroblast. The tank on him must use his or her shield to withstand it.
Fourth, Kael will summon phoenixes that will turn into eggs. The dps must kill the phoenixes and eggs.
Then I think he might die if you do all that. I don't know from experience. We didn't kill him in an hour and a half even after growing to 16 level 80's. This fight is more about coordination and teamwork than your personal gear and level of resist. There really is no level high enough to withstand stuns and mind controls, and those are what get you.