Easy boss disguised by the number of adds he has. This fight is all about execution and skill so make sure you have good pullers and a couple warlocks at least.

Abilities

Anti-Magic Aura: Every couple seconds garr has a aura that removes buffs

Magma Shackles: This is also part of garrs aura, snares everyone to 40% movement speed

Detonation: Garr will randomly detonate his minions causing them to explode and deal a bit of damage

Pull:

I recommend 4-5 warlocks for beginners on this fight, you will need 4-5 of his adds banished and the rest of them tanked. You will have your MT pull Garr off to the wall away from the raid while your hunters/warlocks pull their targets to the center of the room.

Keep a bit of room between the adds to avoid having everyone hit by a detonation.

Fight:

Once you've pulled correctly have your raid DPS down garr's adds one by one until around 3 are left. The adds explode when killed so have melee pull out before it dies, otherwise they get catapulted around the room.

Focus fire on Garr when 3 adds remain, keep those adds banished/warlock tanked. Garr gets steadily stronger/harder as the fight goes on and more of his minions die. Take him down as fast as possible or your tank will be going down.