This peak was done in combination with UN13,291 so the route description for UN13,220 begins from the summit of UN13,291. From that summit, retrace your ascent route as best as possible and return to the saddle just north of the Continental Divide turn. Continue south, heading up the ridge on mostly small rock and gaining nearly 200 feet. From here on out, the ridge is never any more than Class 2+. The only Class 3 work is returning off of UN13,291 to the saddle. Follow on south along the ridge and then west after about a half mile near point 12,990. Earlier in the season when we did this, we found numerous snow cornices and at a number of locations, we could walk on snow if we stayed safely back from the cornice overhang. This may have made the traverse easier overall.
At the 12,700 foot saddle just west of UN13,220, begin your final ascent to the summit along the ridgeline for a ways. It's easy enough until the last 150 vertical feet which presents a rugged, rocky summit block that appears to present some difficulty. At the base of the rocky section, follow a ramp that appears to the left (SW). The ramps takes you out to open slopes which can be easily hiked to the summit. The view from atop offers nice panoramas looking down Marten Creek and the South Fork of the Frying Pan as well as a distant view of the Williams group of peaks to the west.
To return, one option is to hike back east as you came to the 12,700 ft. saddle. When you crossed here, you may have seen remnants of the old Marten Creek trail. You can follow this back down to the SE back to the main trail in the North Fork Lake Creek, but sections of this trail on Google Earth appear to have disappeared through disuse. The other option is to walk down the SW ridge from the summit, at first on broken rock that gives way to tundra, losing about 300 feet or more and then drop down a short couloir, then break out to the SE, making a long, contouring descent back to the North Fork trail. Willows may make this more interesting. Also, on Google Earth, as you head SW off the summit, there's a game trail that's visible, that cuts across the ridge. It appears that this game trail cuts down to the SE across the south face of UN13,220 and then gets lost in the easier terrain farther down.
Links to other information, routes & trip reports for this peak that may be helpful.