While its higher neighbours to the north and south, Lassen Peak and Brokeoff Mtn., attract most of the attention from peakbaggers visiting Lassen Park, Mt. Diller may be the best climb of the three. The peak's SW Face consists of 35° snow when in condition, followed by a short class 3 scramble up a steep, rotten summit pinnacle.
This is an easy but enjoyable climb in winter, offering a relatively alpine feel for a modest drive from the Bay Area and a short approach, and so the route description here is written assuming an ascent in this season. In spring/summer, the route presumably melts out to leave an ugly scree slog behind--and frankly, at this time of year, one can find more appealing options in the Sierra.
Park at the parking area/Hwy 89 road closure just beyond the SW Entrance to the park (see the main page for details), and hike along the road to Sulphur Works. Leave the road here, and make your way up along the west side of West Sulphur Creek. When your path starts to turn west towards Ridge Lakes, leave the creek and continue heading north towards the base of the peak.
An obvious chute splits the southwest face of the peak. It is a relatively consistent 35° snow climb, with a couple of short steeper bulges at the bottom and top of the couloir. Climb the chute to a small notch along the ridge.
The summit is the pinnacle to the left (west) of this notch. Traverse around its left (south) side on a narrow catwalk until you reach an obvious class 3 wall. Climb this broken wall to the summit, taking your time to enjoy the spectacular Cascades rock quality. You'll find a small register (jammed shut when I visited), and some great views.
As a variation, it appears to be possible to bypass most of the chute in favour of cruddy class 2-3 ledges to its left, most or all of the way up to the summit pinnacle.
To descend, either retrace your steps, or descend the easier west or southeast ridges of the peak.
Ice axe and crampons are needed to climb the chute in winter conditions. During a January ascent, I encountered frozen snow throughout the chute, and ice at the very top.