This is a rather technical and difficult fight, which differs in strategy each time the raid instance resets.

Abilities:

Corrosive Acid: 875 - 1125 damage every 3 seconds. Armor reduced by 4000-5000. Hits everyone around Chromaggus.

Timelapse: Freezes everyone in time. Health is reduced by 50% wipes all threat.

Incinerate: 3675 - 4275 fire damage everyone around Chromaggus.

Frost Burn: Attack speed reduced by 80% deals around 1400 damage. Hits everyone around Chromaggus.

Frenzy: Every 15 Seconds Chromaggus goes into a frenzy increasing damage and attack speed.

Skin Shimmering: Chromaggus will shimmer and change his resistances to 4 out of 5 schools of magic damage.

Afflictions:

Brood Affliction: Black : Increases fire damage taken by 100%

Brood Affliction: Blue : Burns 50 mana every second. Casting speed reduced by 50%. Movement speed reduced by 30%

Brood Affliction: Bronze : Stuns every 4 seconds.

Brood Affliction: Green : Deals 250 damage every 5 seconds. Reduces healing by 50%

Brood Affliction: Red 50 damage every 3 seconds. Heals Chromaggus upon death.

Chromaggus Info:

Chromaggus's breath weapons can be avoided with Line of Sight.

His breath is randomly chosen when you zone into the instance.

All Afflictions can be removed by a player except bronze.

(Click to Enlarge)

The alcove shown in the picture is the best spot to tank him as it allows you to use Line of Sight to avoid the breaths. If you attack chromaggus through the wall his breath weapons will then be able to hit you.

Make sure your raid avoids his breaths with LoS each time unless he has timelapse. Then everyone except the main healers and the off-tanks should make sure they get hit with it.

Split the raid into 2 sections 1. MT and healers 2. DPS and off-tank

Chromaggus is tricky when he has timelapse and can wipe you a few times if your not careful.

The MT will loose aggro at some point if chrom has TL(Time Lapse) thus you need 2 off-tanks sundering him from behind. Once the MT gets hit with Time Lapse the off-tank will then be tanking Chrom and the MT will switch with that Off-tank.

Repeat until Chromaggus dies.

You will most likely have picked up a few hourglass sands and you need to distribute them throughout the raid. This is used to get rid of bronze affliction Your MT and OT must have enough to last throughout the fight.

If a player gets all of the afflictions on at the same time he is Mind controlled by Chrom and goes berserk upoin him team mates. Have dispellers ready to get rid of the afflictions.

Since he has the skin shimmer ability mages/warlocks need to find out what he's vulnerable to and inform the raid each time he changes his resistances.