My mage wasn't level-capped when Magtheridon was relevant content. I was a resto shaman, content to spam my one PUG raid night a week spamming four ranks of Chain Heal on other people. I still remember those days fondly, as they were my first exposure to "real" raiding. Also, Magtheridon's Lair was considerably easier than tier 4's other 25-man raid, Gruul's Lair, and the only way to get access to your tier four chestpiece.
I first learned Magtheridon's Lair was soloable at level 85 by managing to do it on my hunter, because pet classes are overpowered (though the ones with self-healing are even more so -- I'm looking at you, Megan O'Neill!). Speaking of potent self-healing, I followed with my blood death knight, and then conquered the pit lord with my enhance shaman. The natural next choice would have been to take a warlock or maybe a feral druid, but I wanted a challenge. Also, Magelam was craving a Crystalheart Pulse-Staff, so he was my fourth choice. I ended up spending about two hours last November perfecting my strategy and have since been able to down him consistently whenever I want a quick 50 gold and some level 70 epics to sell.
I chose to play as frost for this week's kill (which I streamed, though it came out super choppy), but both fire and arcane would have their strengths on this fight. A good Combustion spread might make phase 1 easy as fire, while Arcane gets a 2-minute cooldown on Evocation, and extra healing is always nice. Ultimately, though, I find frost works best. Ice Barrier provides excellent damage absorption, your Water Elemental can tank small damage for you, and you have good, controlled burst on phase 1 adds.
Setting yourself up to win
The exact spec I'd recommend using is this. I maximized Permafrost, as the Water Elemental will tank a fair amount of damage. I also took Piercing Chill, as phase 1 has enough adds that Piercing Chill should help to generate Fingers of Frost and Brain Freeze charges. Mana is an issue due to extensive Mana Shield use, so having Enduring Winter maxed out can help, though you could also skip it and proactively use Mage Armor when you're low. You need a maxed Burning Soul because you'll be getting meleed a lot, and I chose to go with Blazing Speed because there's no other real attractive choice. I'd love Improved Blink, but you can't go far enough into arcane to get it without sacrificing Burning Soul.
Glyph choices are straightforward and all survivability-based. You want Glyph of Evocation to save your life when you're low; time this with an Ice Barrier/Mana Shield to prevent losing cast time from getting hit. Glyph of Ice Barrier is a no-brainer because you want your shield up any time he's close enough to melee, and Glyph of Blink helps you maintain distance and lowers his damage on you. Optimal primes are Deep Freeze, Frostbolt, and Ice Lance. I used Molten Armor and Frostfire Bolt, but those are impractical -- you're on the move so much that you don't get enough Brain Freeze procs to make FFB useful, and you probably won't get much use from Molten Armor. You'll want Frost Armor's mitigation 90% of the time, and the other 10% you'll be using Mage Armor to regenerate mana.
Where you'll die the most
This first phase is the hardest part of the fight. If you pull all five Channelers together, they'll chain Dark Mending casts on each other. If you were fire and could outdamage the heal with an Impacted Combustion, this might not be a problem, but as frost, it's almost definitely a wipe if all five are within healing range of one another. To deal with this, I use my Water Elemental to start the fight. I use the "Move Here" function to position him on the other side of the room from me and set him on Defensive to make sure he attacks people who attack him.
I have my Water Elemental fire off a Waterbolt at one Channeler to start the fight, then have him immediately switch to the next closest Channeler and Waterbolt them. While that's happening, I'm on the opposite side of the room grabbing aggro on my two Channelers. I try to micromanage my Water Elemental enough to have him pick up the third, but that isn't always realistic. Luckily, frost has great burst with Deep Freeze and the two Fingers of Frost charges from Improved Freeze, and I generally am down to four Channelers still alive within the first 10 seconds of the fight.
Don't give up
Phase 2 is when Magtheridon himself is unleashed. The main threat Maggie poses is his melee damage, which you want to do everything in your power to diminish -- I even try letting my Water Elemental get aggro first so that I have a few seconds of breathing room without him beating on me. He also casts a Blast Nova every minute or so, but this can be entirely negated by Mage Ward and is actually helpful, as it stops his meleeing for a brief amount of time. He occasionally casts Quake, which does low damage and is more annoying than harmful, as it will knock you back repeatedly. I generally spam low-damage Ice Lances during this time. He also can Collapse the ceiling on to you in theory after 30%, but I've yet to have that happen.
If you can down phase 1 without getting too low on health, you can kill this fight. Magtheridon himself isn't hard. Mirror Images will buy you 45 seconds of unmolested DPS time; time these right after a Quake, and use Time Warp and Icy Veins for best effect. Make sure you're using your absorbs on cooldown, and remember that you can Ice Block in a bind.
Evocation should be used for its heal and not for its mana regen, as it is your only heal, and Magtheridon is not a short or low-damage fight. Don't Evocate without a Mana Shield or Ice Barrier up, as you don't want to lose ticks of healing to damage. And Blink whenever you can. Any time Maggie can't touch you is damage you're not taking and should be aggressively utilized. Just make sure to never try to use Invisibility as a threat drop, unless you purposely want to reset the fight.
With a careful pull and some serious dedication, Vestments of the Aldor and the Crystalheart Pulse-Staff should be within your grasp quite quickly.
Every week, Arcane Brilliance teleports you inside the wonderful world of mages and then hurls a Fireball in your face. Start out with our recent beginner's guide to being a mage, then check out our three-part State of the Mage columns on arcane, fire and frost. Don't forget to look at some of the addons your mage should probably be using.