[Kevin Sevcik]

A work plane or two is probably the easiest way to do this. They're imaginary planes you create in relation to other surfaces. Like, at X degrees to another plane or face, rotated about a line/axis. Then just make a sketch to cut away all the offending material.

EDIT: Just played with Inventor a bit to verify something. Face Draft is an even better and easier way to do a compound miter like this. It takes two steps, but it's still pretty simple. Play around with it a little.