Well, there are several things you can do as a raid immediately.
As a tank all you can really do is up your effective health if you're falling over to not being healed but ultimately that's only gonna help so much.
Other important stuff (REALLY) is to make sure he has a -ap debuff on him, it has a huge effect on his cleave, which only hits me for 7k at most these days , so that's:
Demoshout/roar/curse of weakness.
Thunderclap.
Ironshields.
Those are the major things you can do.
I really love our Magtheridon tactics.
We have a very group based raid setup and always have, so our tactics do take that into account, which means there's never a moment that all healers are in charge of the cubes.
After a while of struggling we changed to our new (original as far as I know) tactic of this:
Group 1
Me (MT, Add 1/Mag)
Shaman (Resto, WoA/GoA/WF/whatever's needed) (MT Healer))
OT4 (War/Druid, Add 3)
OT2 (War/Druid, Add 5)
Resto Druid or Holy Paladin (OT 4 Healer)
Group 2
Spriest
Hpriest (OT 1 Healer)
Holy Paladin/Resto Shaman (OT 3 Healer)
Resto Druid/Resto Shaman (OT 2 Healer)
Resto Druid/Holy Paladin/Resto Shaman/Holy Priest (Raid healer)
Group 3
OT1 (War/Druid, Add 4)
Hunter
Hunter
Mage
Spriest
Group 4
Mage
Warlock
Warlock
Elemental/Resto Shaman
Balance Druid/Mage/Warlock
Group 5
Rogue
Rogue
Retri Pally
Enh Shaman
Warrior/Feral (DPS/OT3 for the 2nd add).
We set the groups up to be in charge of one cube each.
Each person inside those groups is given a 'priority' to click the cube based on their role.
DPS are always given priority one in their groups, and the lowest DPSers are given the highest priority out of the DPSers.
Healers are given spots based on what they're healing at the time, as with Tanks - Tanks are the only people who get cube priority over DPS if they're free in time to click it (we kill all the adds before he releases, so All tanks bar the MT and OT2 are given Priority 1), which means groups break down like this;
Group 1
5Me (MT, Add 1/Mag)
4Shaman (Resto, WoA/GoA/WF/whatever's needed) (MT Healer))
1OT4 (War/Druid, Add 3)
2OT2 (War/Druid, Add 5)
3Resto Druid or Holy Paladin (OT 4 Healer)
Person 1 Clicks the first flame thingy.
Person 2 Clicks the second flame thingy.
Person 1 Clicks the third flame thingy.
Person 2 Clicks the fourth flame thingy.
Person 1 Dies.
Person 3 Clicks the fifth flame thingy and assumes the role of Person 1.
Person 2 Clicks the sixth flame thingy.
Person 3 Clicks the seven flame thingy.
And so on.
VERY Occasionally you will lose entire groups reorganisation may be needed on ventrilo but this is normally not too much of a problem as #3s/#4s/#5s can take control of the other group as they know they have no responcibility in their own group unless someone dies (and they can reorganise their own numbers when someone adapts).
To finish off the setup:
Group 2
1Spriest
4Hpriest (OT 1 Healer)
2Holy Paladin/Resto Shaman (OT 3 Healer)
5Resto Druid/Resto Shaman (OT 2 Healer)
3Resto Druid/Holy Paladin/Resto Shaman/Holy Priest (Raid healer)
Group 3
1OT1 (War/Druid, Add 4)
5Hunter
4Hunter
3Mage
2Spriest
Group 4
5Mage
3Warlock
2Warlock
1Elemental/Resto Shaman
4Balance Druid/Mage/Warlock
Group 5
5Rogue
4Rogue
3Retri Pally
2Enh Shaman
1Warrior/Feral (DPS/OT3 for the 2nd add).
It SOUNDS complicated, but once it's understood it is insanely simple requires no communication over ventrilo (though you can have someone call '1s' and '2s' for each cube cycle if you want) and is easily flexible if something goes wrong with pre-organised cross group redundancy.
I've never met another guild that has used it, but whenever I explain it everyone thinks it's the best version they've heard.
Tips for 30% (or 20% whichever): You can divine shield the crap from the ceiling as long as the boss is casting he wont de-agro you cuz he isn't attacking.
Leave at least two healers to raid heal and have the rest continue to focus on you.
Stay calm.
Even if you lose people on 30% you can still make it through with the flexibility of this tactic - focus on reorganising within your own groups unless a whole group (or 4 ppl from 1 group) has died - then let the raid leader call for a volunteer 3 from a group or two to take responsibility.
Don't be afraid to blow your timers at 30%.
General advice:
Everything about the fight is designed to make you panic, he is like archimonde.
If you panic, you wipe.
Keep your back to a wall to minimise the risk of you bouncing into healers, and tank him BETWEEN cubes right in the middle so you don't cleave one or the other - don't be afraid to re-adjust his position but tell the raid you're going to do so first.
Have raid healers ready to catch you when he breaks (2-3 should be sufficient to hold you until the rest of the raid arrives from add 4/5).
Try to have at least 3 adds down and no one dead by the time he's out, you may need to bring more healers and double up (healers) on later tanks, but don't neglect your raids hp (leave a healer on it).
Don't move when clicking the cube until he's banished but once he's banished let go - it's not a DPS race so don't kill yourself on the cubes.
Melee are likely to die from his cleave during bouncey stuff if they end up infront of him get them to move out ASAP pop evasion/def stance/bubble DONT TAKE RISKS DPS WILL BE ONESHOT BY HIS CLEAVE.
It is possible, but not practical to stop all waves of his fire nova, leave someone raid healing.
If you're getting him to release every time you're there for the most part.
And yes, Mag is the hardest hitter in T4-5 content (bar Kael's fireballs and Karathress' shamans' windfury maybe), you need to be constantly topped off - the healers priorities is you.
Magtheridon for dummies? (haven't talked about the adds but you didn't mention a problem there)
