The Omnitron trash is simple - separate the adds and don't stand in red stuff, spread out to avoid chaining lightning and kill them one at a time. Magmaw's trash is a little more interesting. The one-pull is simple, just keep his back to the raid and melee but the double pull can be tricky.
We have the adds pulled apart as far as possible while staying within range of all healers, and everyone stacks in a tight clump (except melee of course) in the middle while keeping a line open between the two adds along which they charge the two tanks. Assign one of the tank to be the "damage" tank as in everyone will be targetting whichever add is on that tank, this lets you bring them down evenly.
Make sure melee is not standing in the way of the change but is DPSing from the sides instead to avoid taking charge damage, and if either of them gets to 100k, make sure the other one is also close, have ranged kill one and melee kill the other so they die together. Not too difficult, but can be tricky if one of the DPS or healers are out of position and become the furthest target for the adds instead of the other tank (the adds will target whoever is furthest from them for the charge, thus the positioning.)
2 tanks, 3 healers
This guy can be really easy or really hard depending on how you do it and a lot of folks are still doing the achievement way (kiting adds) which, to me, is a bit more complicated than it needs to be. Here's how I do it.
All healers and DPS go into melee right against his back, while one of the tanks picks up the boss. The second tank stays out and picks his nose until Pillar is cast on him. Have a dedicated healer on the Magmaw tank, another dedicated to the add-tank and a third to cover raid. The nose-picking-tank dodges the pillar, and goes nuts on the adds (I'm typically the nose-picker).
If you need help getting adds down, you can ask ranged to swap and AoE them after a few cycles of Hammer when you have solid threat. I use a full AoE/Damage rotation but if you get too many worms on you before some start dying, don't be afraid to use cooldowns to survive - what else are you going to use them on?
You should be able to go into spike phase with only a couple of worms left and you can burn them down easily on your own and even do a bit of damage on the exposed head (huge hit box, you can hit it easily from range), just be careful you don't get sloppy - if you have any worms up you'll keep regurgitating more and they will agro on healers pretty quick. Keep your distance (stay in range of your healer) and burn the adds - one dedicated healer should be enough to keep you up. The boss goes down super fast as DPS just sits on Magmaw.
For the Magmaw tank, it's pretty simple, watch the Mangle timer and hit a cooldown just before it actually lands as you'll be immobile for a bit while Magmaw noms on you. Remember to taunt Magmaw as soon as he drops you (Mangle/Chains are a threat reset) or he'll wind up one-shotting someone.
DPS and Healers can stay put 100% of the time unless the floor underneath them starts smoking, then you just move to the other side (add tank should move as well) and wait for it to explode before moving back. Make sure you stay within 10 yards of Magmaw at all times. If you have a hunter, it may be easier just to have the hunter with the add-tank to eliminate any chance of a Pillar in the raid clump if the hunter misjudges positioning to be far enough away from Mag's hitbox.
Assign two people (DPS) to be the Chain gang and let them know that they will need to move up to Magmaw's head to be able to jump on him so the assigned people should move up as soon as he slams the ground and click his bony carapice. Let them coordinate over voice chat so you cast chains simultaneously and he should be locked. Hero during exposed head phase and cash in the loot.
Omnotron Defense System
2 tanks, 3 healers (can be done with 1 tank)
You have an option here. You can do this with 1 tank, 3 healers and 6 DPS to make the fight go a hell of a lot faster, but you need a well-geared tank and disciplined DPS who know when to splash damage and when to focus a single tank. On the other hand, DPS is pretty high these days, and you would be safer with 2 tanks. Your choice.
In escalating order of difficulty, the bosses are:
Magmaw: It's easiest if you let the person getting focused stay still and have everyone else move away - that way there is no confusion whatsoever. Just heal through the AoE. Positioning doesn't matter.
Electron: Unlike Magmaw's flamethrower, the target of Lightning Conductor needs to move out and if one of the tanks gets it, they should shout out a warning to the melee to back off a bit. Electrical Discharge doesn't hit very hard but you can spread out if you feel like minimizing damage.
Arcanatron: Move him out of the Power Generator, if you can save Heroism for this, all the better, especially if you don't also have Electron up at the same time to avoid the chains. And interrupt his Arcane Blast. I think every tank class can interrupt at this point. You can build stacks of Power Conversion for mages to spell steal but we don't find it necessary. Maybe if you're bored?
Toxitron: The poison cloud is easy to dodge but people tend to get sloppy with the adds - kiting is important, no matter how many slows you get out. Make sure people who're focused move, I tend to use Avenger's Shield, Judgment and Hammer of Wrath to help out here. Keep in mind that tanks can and will get targeted. I also tend to hold Toxitron in one of the corners of the room and have everyone on the other side to maximize add travel time.
Tanks should watch the shield buff all four bosses get when they get to 50 energy - just wait for it to drop. Once it's off, you can safely move it onto the active boss and let cleaves and AoE hit for more damage (Toxitron is obviously the exception to this, but you can drag him in if your DPS are on the ball with slows, kites and swaps). It's funny how much damage racks up that way, and you'll be able to get it done in, at most, two cycles with two tanks, even less with one.
No real pro-tips for this. Bosses don't hit hard, abilities aren't complicated, I guess you can hit Sacred Duty during Incineration Measures if raid health is low. This is an easy fight - just remind people of the two or three things they nee to remember and don't forget about the deactivating one - especially Toxitron. You will get at least two, maybe three add phases every time he activates, and that kills your boss DPS while he's up. Just focus on them and move on to more interesting content.