General Tactics
Since close-range is where Mag's potential truly shines, most of his gameplan revolves around getting in as fast as possible and not letting go til the opponent's team is toast.
Roundstart
Run Magneto as an assist and switch glitch him in (hold A1/A2 during versus screen) while holding cr.LK makes his 1-frame low come out the second the game starts, forcing the opponent to eat an immediate mixup. This can be beaten with proper timing from the opponent, but this just opens up more layers which is exactly what Magneto wants.
Neutral
If you're having issues getting in, SJ jHP> adc jHP backwards or forwards serves as a near-impenetrable retreat or rushdown tactic, although you want to stick to the ground since that's where his tools are the strongest. Use your air dash/wavedash to dodge assists or your own assist to counter-call the opponent's defensive attempts. Hyper-grav is a good fallout if the opponent keeps trying to fall on you from the air, and on hit puts them completely at your mercy.
Pressure
When you’re close, 2LK>MK> A1/2 either gives you plus frames or a free confirm. Magneto’s tri-jump (sju > ADC df) allows him to force the opponent to guess between a nearly invisible high/low/crossup. If your mix is blocked, use an assist to keep yourself safe and reassert your pressure again; but be on the lookout for reversal assists and push block, if you’re predictable and don’t take your opponent’s habits into account you could eat a nasty counter hit. It’s usually best to take your pressure slow and establish layers—-playing Magneto haphazardly is basically giving the other player a free win and nobody wants that! Once you've opened the opponent up, you can ROM them to death, snap in their assist, or just reset them whenever you want---be warned, it's easier said then done.
Triangle (Tri) Jump
sj -> addf or addb -> attack
This is the basis for the Magneto rushdown game. It gives you a high/low game, as well as a mixup and crossup game.
Uses:
1. Tri-jump in with a sj attack, forcing the opponent to block high. From here, you have a mixup game of either landing and attacking low, or doing another fast sj, thus attacking high.
2. Tri-jump backwards, away from your opponent, with a high priority attack. Allows you to run away and still attack. It also allows you to hit any approaching opponent, giving you an offensive defense, one that you turn into a normal c.lk, c.hp launcher.
3. Tri-jump over an opponent and to catch him blocking the wrong way.
Team positioning
Point
Magneto's best position by far. Using the switch glitch with his 1-frame 2LK can lead to instant, inescapable mixup at roundstart, and a good conversion assist like psylocke puts the threat of a fatal ROM or a snap behind every mixup. Although his Projectile assist is undeniably strong,
Mid
Usually the only time you'll see Magneto mid is in order to absue the switch gltich, but its not a bad idea to keep him here either. His projectile assist covers the entire ground, forcing opponents to take to the skies or get pushed back to the other side of the screen. Not a bad choice depending on your team or the matchup, but evaluate trade-offs carefully.
Anchor
Regulating Magneto solely to an assist character is kind of a waste. Only do it if you really like his assist and you're confindent with him solo.
Picking Teammates
Magneto gets a lot out of the right assist choice. He wants an assist to cover his neutral approach, or to convert into his infinite. Assists that knock opponents like Captain Commando away are less useful because Magneto will have to work to get back in. MSP (Magneto A/Storm A/Psylocke A) is one of the strongest choices for Magneto, giving both a neutral and a defensive assist that both allow for conversions, as well as excellent DHC options. His EM Disruptor assist is great for zoners like Sentinel or Cable, and he can still abuse his strong points with one assist.
Tips & Tricks
Continue Hypergrav xx Tempest into ROM
If you ROM, land, then sj.lk, sj.lk, air dash forward sj.lk, sj.lk HG XX tempest, then after the tempest, you hit sj.lk, sj.lk, land, and start ROM'ing again. In this scenario, Magneto starts falling towards the ground after the Tempest.
On the other hand, if you just do ROM, land, then sj.lk, sj.lk (with no air dash), HG XX tempest, then after you tempest, your Magneto will continue his upward trajectory, so you can't just do sj.lk, sj.lk, land. In that case, you would need to air dash down (or downforward) after the tempest and do sj.lk, sj.lk, land, ROM.
Fighting Magneto
Mashing out of Hypergrav xx Tempest
The perfect and minimum "escape from hypergrav->magnetic tempest" is:
* Hold joystick neutral, towards + FP, neutral, back + RH
IIRC, you can use either strength punch or kick as long it is N,T+P,N,B+K. You can escape as soon as the hypergrav hits. You really need to hit the neutral. Assist buttons never count for mashing. You can accidentally press them, but it's pointless and you should stop thinking they're useful for mashing if you haven't already.