This route is more direct and less rugged than the Southeast Ridge route, but much less scenic due to the Hinman Park Fire in 2002.
Start at the Slavonia Trailhead, and start hiking east on the Gilpin Lake Trail (class 1). At the wilderness sign-in, the trail will split. Take the left route to Gilpin Lake. After approximately a mile, the trail will split again: left takes you to Mica Lake, right (or straight) goes to Gilpin Lake. Go left (north) along the Mica Basin trail that follows Mica Creek, and start rapidly ascending. Continue on this trail to Mica Lake. Up to this point the route follows a well-defined (and well-used) trail.
This area receives little traffic away from the trail. Please follow Leave No Trace guidelines.
At Mica Lake, the point to the northwest is Little Agnes Mountain; north-northeast is part of the Sawtooth Range; northeast are the dual summits of Big Agnes Mountain; and the prominent point east is Point 11777. Do not confuse the summit of Big Agnes with Point 11777. On the other side (east side) of Point 11777 is the scenic Southeast Ridge route.
At Mica Lake, leave the trail and head east towards the saddle between Big Agnes and Point 11777. As you approach the steep slope of the saddle, a fairly obvious grassy slope to the left (north-northeast) leads to the south summit of Big Agnes. The grass will turn to rock as you get higher. Depending on your route selection at this point, the climb will be either class 2 or 3. There are two "summits" on Big Agnes. Last time I was there I forgot to look at my GPS on the southern summit.
For added complexity and class 3 scrambling, continue to the top of the saddle, and traverse the ridge north to the summit of Big Agnes. There are significant drop-offs to the right (east), so favor the climber's left side.
None, besides the ten essentials and common sense. If you are planning on spending a significant amount of time on the rock east of the summit, you may want a helmet.
Have fun! Expect to see few people (if any) off the trail. The summit of Big Agnes gives you an excellent view of the area, including the Diamond Park, Red Elephant Mountain, the Sawtooth Range, Mount Zirkel, Flattop Mountain, Lost Ranger, The Dome, Mount Ethel, and Hahns Peak.