Yeah, don't use a pally on the main named unless you want them to possibly top the heal parse by a lot. Charm usually happens shortly after the snake adds spawn.

From the time the adds come and he says "Come my children", you have about 30 seconds to kill the adds. If you fail the dps check, you will get the message in red letters across your screen "The volatile slitherers become unstable and explode." Raid will wipe unless you have blockers up.

Honestly, we put out more than enough DPS now to kill the adds, its just faster to ignore them, burn named, and AE block the explosion. That is if you have enough AE blockers. Other than that, the above listed strat is also perfectly sound.

Strat: -
Named will periodically charm the MT.
Must have 2 tanks on named ready for this (Neither of them a Paladin, unless you want to have him % base heal the named)
Will spawn large groups of snake adds, that are a dps check. If not killed quick enough, cause a large fatal AE that is usually a wipe. Roughly about 1 Min between adds.
Let tanks stack adds nicely before unloading dps and you will get a much nicer DPS burn.

Nothing more to the fight really, just a pure dps check.

18 million is not going to be enough imho. If you cant put out about 22mil or better AE DPS, you are going to have to go with the AE blocker technique. This is honestly faster anyway. You never have to stop burning the named and it minimizing the risk of loosing your tanks.