Once you get past the Unholy Temple, it's a cakewalk. The last level is a little more challenging, but if you have the Halberd from one of the levels after Unholy Temple and a decent amount of Rage Capacity/Rage income %, it's not much of a challenge.
Ahh, thanks to MakotoShinobi:
To earn the medal that requires you to beat any level with only shoulder weapons, remove your primary cannon, and leave the shoulder weapon being equipped.