Terrax and jobbing are so synonymous that if you now removed the element of jobbing from a comic-book depiction of Terrax, you would have to re-introduce the character as nobody would recognise him!
Also, Magneto seem to overachieve whenever he faces off against higher-heralds.
Theoretically, the Power Cosmic includes control over electromagnetism, in addition to the other fundamental forces, so Terrax should have the advantage. But Magnus seems much more highly competent than Terrax.
If Terrax doesn't blitz right off the bat, then Magneto takes 6/10.
Terrax's standard power like one shot large planets with a casual unamped swing of his axe, transmutate objects and drain energy, move at FTL speeds. Wins in a stomp.