This is another area classic, and the easiest way up Magnum Block. Start on the same triangular hold as Magnum Force, but move right hand to a hold underneath the eastern arete. Pop a left heel-hook on the starting hold, then move your left hand to a hold along the lip underneath the western arete. Right hand to a horizontal edge up between the aretes, and left hand up to a hold along the western arete. Grunt your way to the end of the aretes, and traverse off to the left using holds on the underside of the roof.
South-west underneath to southern arete.
Spotter for sure...a pad may be of some use, although it reduces space underneath.