These guys are a walk in the park compared to Freya.
Easy mode is kill biggest to smallest and hard mode is smallest to biggest. Obviously we did easy mode first.
You will need 3 tanks - each of them drag their add off out of range of each other. We found that anything other than a bear able to tank Sarth 3D was getting oneshot on the big guy with our current gear level (ie mostly i213 with i226 highlights), so highly recommend something like this:
Big guy - bear
Middle guy - Warrior
Small guy - DK
The big guy puts a NASTY debuff (edit: our bear tank decided to call me on my terminology. This is apparently a magic debuff. But you lot knew what I meant, right? You have all read the technicalities of the fight on wowwiki or something eh?) on your bear which needs to be decursed (edit: *cough* dispelled by a pally or a priest, happy HB?) INSTANTLY. Make sure you have an allocated decurser watching the tank's cast bar. They should be spamming the decurse as the big guy casts his "Me squash tank now" spell as it starts ticking within a second! Master that and it's just tank and spank.
The middle guy throws out some serious damage but nothing that a Naxx25 holy pally cant solo heal with a bit of mana management. Make sure that if the tank moves the boss that he let's the healer know. Once big guy is down, we rotate tanks. Bear tanks middle guy and warrior heads off to help on little guy. Middle guy is again just watch for aoe (RUN AWAY LITTLE GIRL! RUN AWAY!) and tank and spank.
Little guy needs constant interrupts on whatever spell it is he does. When he is the last man standing he will periodically turn into a flying electrical octopus and totally pwn anything he touches. RUN AWAY! We dont care where (the tanks will follow and be ready to pick it up afterwards) just dont trap yourself in a corner.
Through all this, there will periodically be blue and green "fire" on the ground. Keep the bosses OUT of all this stuff. For you, Blue is good, green is BAD. (why couldn't they make it the other way aroung - cos alliteration makes things easier). So stand in blue for DPS buffs, and run out of green (or die).
Then the last boss is at about 10%, quickly suck down an iron boot flask and get yourself an extra achievement!