I am a 25 man raider however I have done Heroic Immerseus on 10.
If your paladin can't manage his heals tell him to spec out of eternal flame. Its not like tank damage is going to be a problem on this fight.
Spec in to bladestorm for this fight. Bladestorm is your default ability to pick up adds with mocking banner used whenever its off cooldown. Make sure to use some active mitigation as well as a personal cooldown before you bladestorm as you will be taking a lot of damage.
Just after the adds spawn Immerseus will do sha bolt and drop those pools on the ground. This is your que for the raid to stack on you. You will have 10+ seconds of free time before he does sha bolt again. This will be plenty of time to pick up the majority of adds with blade storm. Look at your raid timers and make sure everyone spreads out before the next sha bolt or it will be a short lived fight. Stacking will also allow you to maximise usage of mass dispel which is really helpful as the debuff you get from damaging the boss is much more of an issue in 10 man.
Once everyone has spread pick up the last few adds with your normal abilities.
You might think the adds are your primary target here but they are not. It is more important to get those swelling corruption stacks off the boss. Once they are gone blow heroism/lust and use all dps cooldowns (have your dps save them for this point). The goal is to have a transition before he gets his second swelling corruption buff. Make sure to tank the adds next to the boss for max cleavage + aoe near the boss.
You will need to pick up the boss after the second Corrosive Blast. Your paladin tank won't survive a third.
The most difficult part of this fight is the start. As you survive each transition the fight becomes easier, towards the end its a joke. Don't be afraid to front load your raid cooldowns as you won't need them later in the fight.