When they put an addition on my house they had to do some hairy compound miter cuts to fit the new rafters to the old roof. We're talking 2x6s where the cuts are like 15" long.
They did the cuts freehand with a circular saw.
For just one cut I'd mark it with a straight edge and cut it. Or use my circular saw

Most mitre saws only go to about 47 degrees because mostly you don't need a lot of cuts that go further. If I was doing a project that required a whole lot of 60 degree cuts I'd probably make some kind of jig that would let me put the short end of the stick against the fence and cut at 30. Unless I was working in my shed, and then I'd use the circular saw

Bob