Start at the north halfmoon creek trailhead, which is 2.5 miles beyond the Mt. Massive trailhead on 4x4 road #110. This trailhead is right before a creek crossing.
This route is the same as the west slopes route described in Gerry Roach's Colorado's fourteeners, except for which scree gully you scramble up. Hike on class 1 to Halfmoon lakes, then follow cairns northeast to a glacial ampitheatre at 13,000 ft with huge erratics. This is a great place to camp! To the north you can see the summit of North Massive and directly east is the summit of the mass of Mount Massive, which is indistinguishable from the rest of the ridge.
Massive Green Ampitheatre
Between North Massive and Mount Massive is the pink, granite backside of Massive Green which makes up the ampitheatre. On the right (south) of Massive Green there is a lovely scree gully with about a 35 degree pitch. About one third up this 1,400 ft gain you can take two routes. You can traverse left (north) towards the saddle between Massive Green and Mount Massive and then hike the ridge to the top. Careful though because the first summit is not the true summit, you got to go one more over. The second option is to challenge the ultimate stairmaster and keep going straight up, staying right (south) of a triangular pinnacle of dark rock and staying on large scree. This means you will be going up the left side of the scree gully, avoiding the wash out. This will take you almost to the top, hike to the right (south) about 60 ft. away and maybe a 20 ft. vertical gain to the top!
West Couloir route
It's quite enthralling to climb this and find that you're already to the top! When this route is viewed from the ampitheatre at 13,000 ft. it looks too steep because of the light colored wash out, but it has been climbed! Decend down the west slopes or southwest slopes route.
Lots of food and water, good boots, binoculars, camera and a light pack.