I think Ao100 is harder than the main game at equivalent dlevels, as without the special levels, you're weaker by about 1-3 levels and lack all the nice guaranteed loot from them, leaving you a lot less prepared when the elite enemies start showing up and making you especially vulnerable to bad RNG in the 6-20 dlevel range. So I do find it unusual you succeeded with your first HMP win on it not only with AoMr, a challenge that leaves you especially weaker in the early game, but also as a Sharpshooter, a build that has it notoriously hard in early levels before you get the master trait. Though finding good exotics compatible with your build early on helps a ton in getting you over the big hump that is the first 20 levels of Ao100.
Also even if you did kill everything, you wouldn't have reached level 25, you just don't get enough experience on HMP or even UV to reach it in 100 levels (unless maybe you're on UV and get many levels containing only boss monsters). You have to play Ao100 on Nightmare, play Ao666, or dual-angel Angel of Darkness with Ao100 to be able to reach level 25.