Since we have included a backpack approach for these summits, the "route" begins from where the West Maroon Trail makes the first crossing from the west to the east side of the creek. When we climbed these summits in 1993, the winter of that year was a high snow year in the Colorado mountains. Those heavy snows persisted well into the summer, so our route decisions were influenced by those conditions. The three summits south of Pyramid were all climbed on the same day which was July 23, 1993. Most rate this a Class 3 climb on the assumption you stay mostly in the couloir. The difficulties we encountered staying out on the rock rib are what cause us to give this a Class 4 rating.
From the vicinity of the suggested campsite, locate where the trail crosses to the east side of the creek and make the crossing however you can. There was no log bridge of any kind at the time we did this. On this occasion, there was a snow-bridge that made crossing easy. Once across, head east up through trees, working your way along by finding whatever trails you can - some game, some man-established. There are an alternating series of cliffs and benches with areas of trees. In 1993, we found a fairly direct, cairned trail that led us through a boulder field, then alongside a steep, snow-filled gully to another bench area. From there, the route into the upper basin at about 11,800 feet opens up rather naturally. By the time of this writing, we would expect there to be a well-used/marked trail.
Once in the isolated upper basin, there are any number of possibilities for heading south, up the basin. Since this area faces north and is fairly sheltered from the sun, the snow in some areas may not melt out until later in the summer. The general rule is that the west side of the creek has more tundra while the east side is almost all talus. Since you'll have plenty of loose rock/talus to deal with later on anyhow, head up the basin on the west side of the creek making as much use as you can of areas of tundra and stable rock. The real work will begin when you reach about 12,100 ft. This is when things steepen and the tundra begins to play out. If you arrive here earlier season, be prepared for snow and bring an ice axe. The snow can be a big help in avoiding so much rubble and scree. Above 12,400 you'll want to swing more to the SE and aim for the saddle between Pt. 13,586 and UN13,631. At one point we encountered a brief ramp of small boulders and talus. They tended to be covered with lichens and moss - a deadly combination if wet. The final 400 feet of ascending to the 13,300 foot saddle will be exclusively on small boulders and scree - an unpleasant combination for secure footing of any kind.
Once at the saddle, turn south and follow the ridge to the summit. There will be some small cliffbands to navigate through or around, but generally you can stay on the the ridge and keep things at a harder 3rd Class with no real obstacles to slow your ascent. Much of this route up we found to be cairned in various spots. Enjoy the amazing view of the Bells and surrounding peaks, then begin the trip back down. The descent from the saddle will be loose. We found ourselves often sending small boulders careening down the mountainside. If there is snow, enjoy a speedy glissade back down. Pick up your trail coming up from West Maroon Creek and retreat to your campsite or make the long trudge back out to Maroon Lake. As we indicated on Thunder & Lightening Pyramid, we actually completed all three of these summits in one day, but had not planned on doing that. From the suggested backpacking campsite, we ascended into the upper basin, went up Lightening Pyramid, traversed to Thunder Pyramid, descended back into the upper basin, then ascended UN13,631B, followed by a hasty retreat back to the campsite because of weather closing in. Doing all three was a long day affair with over 5,000 vertical feet of gain.
Just for your amusement and to demonstrate that the weather in Colorado can do anything it wants, any day of the year as we tell friends, once we completed the three summits this day, we went back to our campsite and as we headed for bed that evening, a light mist was falling that continued through the night. Didn't think much of it, but the next morning when we finally arose and had enough light to see, from about the 11,000 foot level up, all the peaks were covered with about 2 inches of new snow! Remember, this was July 23. You never know what the weather is going to do.
Links to other information, routes & trip reports for this peak that may be helpful.