I'm directing this towards your reply, because you're our resident expert re: your namesake.
I actually think close quarters might not be good for Mandarin unless its very close and Magneto doesn't just automatically have his shields up. Mandarin is fast and skilled and can probably get to Mags before a shield goes up - if he isn't shielded already and they are within striking distance of one another when the fight starts. At even five feet, I think Mags could usually get a shield up before Mandarin could strike him despite Mandarin's speed.
On the other hand, though, the magnitude of the rings' power and the variety of effects they offer seem to me to give Mandy a better shot at getting past Mags' defenses than most.
Well the thing is, Magneto gets hit a lot despite his shields. Even metal-boned Wolverine hits, and in fact has nearly killed him on multiple occasions. He's not really that consistently fast about getting the shield up. He's alive because the X-Men aren't killers other than Wolverine. So no, I don't think Magneto can get his shields up fast enough if the fight starts close up. And unlike the X-Men, Mandarin is a killer-born, something that isn't really quite true even of Wolverine, and isn't remotely true of the X-Men in general. If Mandarin lands a punch with Magneto's shields down, he will put his fist clean though Magneto's head or heart.
As for what he could do to counter Magneto's powers if he does in fact get the shields up, I suppose he could teleport Magneto somewhere, but Magneto may be able to simply open a wormhole and teleport right back. I suppose he could use psionic illusions to trick Magneto in some way, maybe make him think he's already killed Mandarin so he drops his shield. Magneto has an iron-will, but his record of resisting being knocked off balance by psionic attacks isn't nearly as good as his record of resisting being controlled outright.
But really his best bet is that Magneto isn't as quick about putting the shield up as some people seem to think. He also sometimes drops it mid-fight because he's concentrating on something else.