Kinda new to heroclix, obviously. I play big games with a friend 1000 pts or more. This magneto is a thorn in my side. Only beat him once in the past. And that only happened because there were 3 of us were playing on a large map that had a force field on it. Myself and Buddy ganged up on third friend to take out magneto. Furthermore, I stood behind the force shield with cosmic spiderman hammering the hell fire guard. And then we both turned on magneto. Not going to have that opportunity again. It will be 1 on 1 this time. Please help.

His unassisted range is potentially 16 unless I read his card wrong (start on debris, running shot for 5 square movement, end on debris for 11 range). That means his TK reach is the entire map, excepting terrain.

Barrier is how I answer being out ranged. It will work here as well.

Hypersonic Speed will put you in into melee with him instead of ranged combat. TK will close the gap on him fairly quickly unless he stays in his starting area (most HSS figures can move at least 10). You'll need a 12 attack to reliably hit him even up close. I don't know what you have available but most probably you need to add some perplex in to get extra attack. If possible you'll want super strength.

Assuming I had no figures and a budget of $5 I'd use this (300 pt modern age):

wxm005 Oracle 49
tt007 Jinx 39
wxm017 Jean Grey 40
tt012 Red Star 120
dp018 Blind Al 13
fl020b Mirror Master 30

Barrier in forward positions in order to safely position your team. If you can stay in the starting area Blind Al provides an extra perplex. At the start of your turn the barrier comes down, the perplexes go to red star's attack value, TK (action 1), Hypersonic (either already holding an ultra heavy, or grabbing one as you move) and attack Magneto, use any remaining movement to move to safety (this might be an all in attack depending upon timing an positioning) (action 2), Barrier (action 3).

In an ideal situation you'll be able to TK, attack and then have enough movement to get behind the barrier again. If the game is larger you can bring an extra TK (TK out, HSS, TK back) and more main attackers.

Blind Al can't perplex out of the starting area

Hmm, you're right. I will fix the method presented above.

That's still an 11 attack (12 if you start in the starting area, which still gives 19 reach because of adjacency), potentially with two PC backups depending upon the situation. She's still worth including though.

*If the game is going to be more than 300 points and not modern age restricted, Klingon Engineers (which can just be printed), are better options than Blind Al anyways.

Also knowing what you have access to will help us give more specific advice rather than general advice with an example.

I would wait until Mags is pushed, then move in with Shatterstar carrying another nasty piece. They will make quick work of his Mastermind fodder.

Also if he using the Hellfire Guard-derived Mastermind to stay alive, Precision Strike would work well. A TK'd Cyclops bomb does a number on many figs.

Another idea would be to use Angel from DoFP (with Sidestep and his speed value he can move/taxi 10 squares) and have him bring Nightcrawler with him. If you give them the X-men Gold ATA one of them can declare Mags the target and then Mags' stats cannot be modified. So no ES/D, no hindering terrain bonus, etc.

GSX Magneto was king when the set came out. Now he's good but there are many ways to hurt him. Get into melee with Mags and use figs with Precision Strike to bypass Mastermind. Or you could Mind Control (Broother Voodoo?) him away from his goons.
Use Precision Strike figs like:

wol025 The Weaponer of Qward 211
catws017 Captain America and Black Widow 130
ffjltw001 Ultraman 250 / 150
fl048 Apollo 200
dofp013 Wolverine 130
jltw038 Hawkman 115
dp036 Bullseye 110
wkD-020 Cheshire 105
dp047a Shang-Chi 90
dofp009 Nightcrawler 80
gotg028 Karnak 75
dp003 Dogpool 65

Poor Teamwork.