This can be a very annoying boss fight

Thekal Abilities:

Heal: Thekal can cast a heal spell upon himself and the Zealots.

Ressurect: If Thekal and his 2 zealots are not killed wihtin ten seconds of eachother they will ressurect much like core hound packs in Molten Core.

Tiger Summon: When Thekal is in Tiger form he can summon tigers to aid him.

Charge: When in tiger form Thekal will randomly charge another member of the raid knocking back and hurting everyone in his way.

Knockback: When in tiger form thekal can knockback whoever he is attacking breaking aggro.

For this fight you will need 3 tanks, 1 for Thekal and 1 for each of his Zealot's. The zealots and thekal should all be pulled away from eachother to prevent healing. Thekal can hit hard so make sure all your healers are ready to heal the Main-Tank instantly.

What you need to do is to Focus fire Thekal down to 10% then one of the Zealots, then finally the last Zealot until all are at 10% then kill them one by one as fast as possible and hope that you do it within 10 seconds.

Once all are dead Thekal will revive in his Tiger Form. This wipes his aggro list so the Main-Tank needs to be on him immediately. You can decide whether you want to drain his mana so he won't summon his tiger minions or not.

You should assign one Main-Tank to him one tank to bring Thekal back when he charges and one tank to take the tiger adds. If your tanks are able to keep Thekal away from the raid you should be fine and just burn him down.