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.