There are two major changes for heroic mode: (1) There are blazing skeleton adds that spawn one at a time. They must be tanked and it is usually the melee's job to kill them. You have to have a fair amount of dps on them otherwise at 20% they start casting an uninterruptable Armageddon and will deal massive raid damage if they don't die. (2) At 30% the adds will stop spawning and Nefarian will start throwing shadow damage at the raid. It hits random sections of the room so everyone spreads out to lessen the chance of getting hit. I usually sit in the middle, pop shamanistic rage, drop a glyphed Stoneclaw totem, and wail on him.
If this is 10m it's most likely that it will only be you and another melee on the adds (along with a tank). I've been paired with a warrior, a rouge, even a shadow priest, and we've never had any trouble getting the add down. The add crashes down as he spawns, but it should be in ranged. He spawns fire with him, but it shouldn't wander into melee, just keep an eye out if it does. However, it is also melee's job to jump up on the head and chain Magmaw to the spike. This is priority number one! Drop everything and jump on that head the second the vehicle command becomes available. Be sure someone calls it out too because ranged will have to switch to killing the skeleton add because Armageddon almost always occurs 2 seconds after you can jump.
I'm assuming that if you're on H.Magmaw you've been on chain duty for normal mode. It's the same deal. Hope up. Press 1, aim green circle on spike, click, chain, profit. If for some reason you're not on chain duty, for the love of loot, do not try to jump up. You can glitch it and throw the designated jumper off. This makes for a very, very unhappy raid.
Basically the only time you're dps-ing Magmaw is when his head is down. The goal is to have Magmaw down to 35% after so many mangles (around 3 maybe?) so that when he is spiked that next time you can pop hero and get as much damage on him as possible so that the Shadowflame Barrage phase is short. When you're on that last dps-race you can ignore the skeletons and just focus on the boss until he's down.
Other than the above, it's the standard fight. Save your wolves for the spiked phases, use damage cooldowns when appropriate (sometimes not needed but your healers will thank you).
If you thought normal mode had a lot going on, it only gets worse. Welcome to the cluster---- that is Heroic Omni.There are still two robots up at any time, they still do all their normal skills. However, because Neferian's only entertainment seems to be making raiders lives harder (really, can't the guy read a book or something?), now all of the skills have a chance to do something extra.
Arcanotron: Power Generator now has a chance to swell and blow up. If the circle is growing, GTFO! (And laugh at the mage who dies because he "thought he could blink out in time")
Toxitron: Neferain can choose to drag the entire raid into Chemical Cloud. Run out, but make sure not to hit any adds on the way.
Magmatron: When he "acquires a target" and hits them with Flamethrower, there is a chance that the person will be rooted in place. If your raid is used to the targeted person removing themselves from the group, now is the time to learn to remove yourselves from his presence instead. Just pop one of your damage cooldowns and pray your healers focus on you.
Electron: Lightning Conductor will sometimes turn into Shadow Conductor. With lightning conductor you want to be spread out, but it's the opposite for shadow. Once it turns, everyone should stack in on the affected person to mitigate the raid damage.
Neferain only tampers with every other attack. So if Magmatron and Arcanatron are both up, and Magmatron's spell was just buffed, Arcanatron's pool should be normal. The key to this fight is just learning how to deal with all the buffed spells.
As in normal it will be your job to interrupt Arcane Annihilator whenever Arcanotron is up. If you have a mage in your group - one that you trust - you can play the very dicey "let Converted Power stack a couple times then have the mage spellsteal it." We do that and it makes my mage very, very happy. It gave me heart-attacks the first few times because I wasn't sure I could get all the interrupts. If you and the mage work well together, go for it. If not, I wouldn't risk it. On 10man I can get every interrupt, on 25 the tank and I swap them.
That's all for now - I'll probably update with some doodles later. Everyone's favorite mad scientist (no, not Professor Putricide, the dragon-looking one) up next.