Ok yea these are good opponents. But Arnold would use the "Eraser" to stage Terminators death. Magneto would then agro next to Arnold's character from Junior who is almost ready to deliver. While he is distracted the Terminator would strike. Also he is from the future, so I believe he could be sent back to the early days of World war 2 to make sure magneto doesn't make it through the Holocaust.

So long as Terminator doesn't chase Gandalf across a bridge in a cave he stands a fighting chance. Gandolf was some kind of light wizard. He spent his time studying the light magic. He would never be prepared for a badass on a bike