I am almost done farming the popset. I can't speak about killing Proto-Ultima, but the entire popset can be farmed solo on BST for sure. Parts of it were very easy, and parts of it were somewhat challenging. I can say that I had a blast doing it solo. Enough challenge to keep me engaged, but not so difficult that it becomes frustrating. I also socked away items to upgrade relic gear for every job I play while doing it. Made some pocket change from selling ABC's as an added bonus. The fact that you can enter every day now instead of every 3 days makes this far more realistic to do, as well. Go for it. You'll have a lot of fun.