25-man is incomparably harder than 10-man, but that's besides the point.
We got the kill rather quickly in about 2 hours from our first pull (we went as a fun night on an off-day from 25s), and this was even our first time pulling the boss on hard mode, even before our 25-man kill. If anything's going to stop you from killing the boss it's frustration and repeat errors. Also, you don't need an obscene amount of stack resets (though it helps) or four healers.
What we went with ~
(tank) Paladin
(heal) Druid
(heal) Paladin
(heal) Shaman
DK
DK
Hunter
Moonkin
Arcane Mage
Here's a diagram for P1 positioning.
Red indicates the tank
Purple indicates a healer
Green indicates DPS
The numbers 1-3 in each of the segments represents the danger level. Areas labeled 3 being most dangerous, and 1 being least dangerous. You'll want to put all of your least-mobile people (such as melee) and stand-still healers in 1 spots, and highly-mobile people (such as hunters) and movement-enabled healers in 3 spots.
For P1 you'll want to isolate the tank as well as his healer in the spot next to him. When a fork lightning appears, have one person move from it and the other person stay inside of it. This will lessen the chance of spreading forks to 4-people groups. If a fork happens to spawn on a healer group and the double dps group he is also responsible for, don't be afraid to use cooldowns. They will be back up before the fight gets hectic again in P2.
For P2, group up on the tank healer's spot and regenerate mana. The beginning of P2 is very relaxed. You'll want to get out of the phase with everyone at roughly 18 stacks in 10-man, 19 at the absolute highest. At this point, the raid will be taking more damage than you could heal bomb back up. Though our group had many stack-resetting classes, this wasn't very necessary as we were transitioning before we even hit a 15 stack. Let the tank take the adds and dedicate one of your ranged to handle adds full time for debuff juggling. We used our hunter for this. Make sure the person is vocal as there are times where they'll need a bit more burst to beat the timer.
For P3, you will group up at the head and go top-top-top, bottom-bottom-bottom, top-top-top, bottom-bottom-bottom (rinse and repeat). Once you get into phase 3, just don't do anything stupid and it's a guaranteed kill. Nothing at all changes with the exception of cloud spawn rates and damage output.