Actually taking out Mastermind isn't that impressive...he's just a human.
I actually have 15 and 16 as part of a trade paperback. I just now dug it out. The line up is Nighthawk, Hulk, Strange and Valkyre. Xavier comes to them for help in beating Magneto and the Brotherhood. They go to help. Mastermind creates orange monsters and Val, Hulk, Nighthawk and Strange are attacking it and nothing is working. Xavier tells Strange to calm down, it's an illusion. Strange doesn't believe Xavier and keeps attacking. Even though it's technically an illusion, they're all acting like they're getting beaten up (don't know why). Xavier, who gave up trying to convince them, uses his power to disperse the illusion. They continue when suddenly they're all hit out of nowhere by Magneto, who knocks them all out.
They wake up a bit later, trapped in a magnetic field Magneto erected around them. Magneto explains some long plan to build Alpha, he does a bunch of "last time in my life" exposition to explain what he's been up to recently, then leaves and goes about his business.
Cue the Deus Ex Machina from Xavier, and they're able to combine their mental energies through Xavier who telepathically shuts down the field (????). Magneto senses what happened and send his Brotherhood to take them out. Hulk vs. Blob, Strange vs. Mastermind for round 2, Xavier and Nighthawk vs. Loreli. Strange STILL falls for Mastermind's tricks, and starts getting fatigued because nothing he's doing is working and he's too paranoid that one monster might be real. He's using something called the "Flames of Faltine" to dispurse the illusions, but more keep coming. So eventually he makes illusions of himself, which confuses Mastermind, and then Strange blasts him. Everybody else wins their fights. By then, Magneto has finished Alpha. That's #15.
#16 is where they get owned by Alpha. Magneto buried the Defenders under rubble and used Alpha to teleport himself and the brotherhood out. The Defenders dug out eventually. They track down Alpha and Magneto to the UN, where Magneto tells Alpha to destroy them. That's when Alpha refuses and blah blah blah, you know the rest.
Sorry I don't have a scanner, but I assure you that's how it went. I hold it in my hands! 🙂