Umzok is pretty straight forward. Joust his Trauma aoe and get that DPS up. Umzok pretty much needs to die within 5-6 minutes.
Upon killing Ozyk you will notice that you will get a Shard of Reversal Energy from that ghost ogre near the entrance to the next room. This shard needs to be used on the Epicx4 adds that will arrive with Umzok. These mobs spawn based on time from when the first add dies. The reason you want to kill these adds is they increase your dps when you use the shard properly. If the add is not killed within about 60 seconds it will buff Umzok's dps. If you miss 2 adds, the buffs are too great and Umzok will begin one shotting everyone.
You need to use the shard just before the add dies. The reversal energy ability lasts for 8 seconds, so based on your raids dps you might want to use the item on the add around 10-15%.
Here is the kicker, the adds after a certain ammount of time begin spawning too rapidly for your raid to handle. So, this is where the time comes into play.
The way the pattern for the adds plays out is this.
Add will spawn on incoming (kill it)
<time to burn>
Add will spawn (kill it)
Add will spawn (kill it)
<time to burn>
Add will spawn (kill it)
<time to burn>
Add will spawn (kill it)
Add will spawn (kill it)
<very very little time left to burn>
Adds get out of hand (you die)
You pretty much want to aim to have Umzok killed by the time the 4th add is arriving. The reason you want to aim for this is because you will have even less time to kill Gozak once you kill this mob effectively. So practice dps on this mob in preparation for Gozak.
The last thing about this encounter is that the adds will mem-wipe. So you want ideally atleast 2 very good hate machines on the adds.
DPS like champs and Umzok is a little baby.
Good Luck.