I haven't done this on 25man yet, this is just my opinion of 10man.
Pretty easy fight(no surprise there). First 2 attempts we had a hunter dropping ice orbs to keep the boss from submerging. After 2 wipes I noticed that no matter how hard we tried we couldn't keep the boss from going under. This caused low dps which led to the enrage timer. So we scraped the whole "keep him from submerging" idea.
Third attempt we had our warlock put dots on the orbs near by and didn't bother trying to keep him above ground. This provided a lot more DPS and allowed us to down him with ease.
Keep a few ice spots down in phase 1 for the adds. If the adds submerge they heal to 100%(2 adds stacking debuffs on the tank can lead to a one shot). When Anub submerges everyone group up on the ice and have 1 person shoot down the orbs as much as possible. Move to a new spot when Anub tries to impale the raid. Try and stay together so the tanks have an easier time grabbing the adds. Hunter's aspect of the wild is very helpful in this phase.
When he gets to around 35% make sure all the adds are dead. Once at 30% he starts leaching health from the raid. Try and keep everyone except the tank around 50% HP to keep Anub from draining too much health. Adds stop spawning in this phase and he no longer submerges.
we cleared the 10 man last night very easy fight i though , we had a hunter killing the orbs and just tended to stay in the permafrost during the spikes i was OT on the adds which we didn't tank near the boss but tank just away from it and a lock and hunter took them down.
Tried the Hardmode after that was hard we have to rethink that and read up on it abit and we hitting it again on Friday