Fantastic Four vs. Brotherhood of Mutants
*The Fantastic Four as of Rise of the Silver Surfer
*Brotherhood as of the Last Stand
*Magneto has feats from DOFP
*Starting Distance 70 feet
*Setting Downtown NYC
Round 1: Phoenix isn't in the fight
Round 2: Phoenix is in the fight, but Doctor Doom comes to aid (With Silver Surfer's Board)
vs.
With all the metal in downtown NYC, Magneto could likely solo round one, as others have said. Thing could be hard to hurt without his Apoc power amp, but he can just wrap a metal pole around Ben and BFR'd him, similar to what he did to Logan in DoFP.
I'd go FF via Doom for the 2nd round. Not only did he have cosmic powers great enough that he could blast chunks of mountains apart, create giant whirlwings/tornadoes etc., he also had FAR greater physical durability than Phoenix.
He pretty much lost due to PIS in that film. Johnny 4-in-1 couldn't do any kind of damage to Doom with his fire, strength etc. (seriously, he couldn't even singe Doom's cloak). But miraculously managed to damage the wrist controller Doom was using for the board, which should not have happened at all.
It's hard to know how much was metal. He seemingly returned to flesh after getting blasted by Surfer, yet retained his durability/electrical powers (we never see him do much physically, but it can be assumed he also retained his strength). Assuming he is mostly metal, Mags could possibly take him out. But Doom has more than enough power that he could turn Mags into paste with little more than a hand gesture too (he crushed a piece of mountain by clenching his fist at one point, IIRC). And Magneto would have to know to specifically target the wrist controller. I personally feel like Doom would beat him to the punch, not being old and geriatric.