It's best to do this achievement on heroic. Your raid group will already be split into two groups making it easier to spread out. Best way to position the boss (without pull/clearing the side trash) is to have Morchok near the entrance to the temple and Kohcrom near the instance portal. That should give your entire raid a chance to spread out.
Tanks can also survive Black Blood of the Earth with CDs, so they don't have to move, giving you one less person to worry about getting near you.
Make sure to track the achievement: if you fail, the tracker will turn red.
The boss has 130 Million HP and CAN be completed in a raid group. You will need at least 1 tank and 1 healer. Depending on the DPS you bring, it's mostly just a tank-and-spank. If you are having problems finding a group for this, openraid.us/openraid.eu have groups forming for it.