The Mundus fight is infinitely better than the Savior fight. While mechanically, the Mundus fight is straight forward, it's visually impressive, and what you need to accomplish is straight forward. Mundus has a few attacks that do a lot of damage, and it naturally includes platforming, evading, parrying, and proceeds the story in an eventful fashion.
The savior fight in DMC4 is just confusing, it relies a bit too much on guns, and having a boss fight where it's unclear on what the fuck to do is just bad. What's worse, is that the Savior fight in DMC4 does what I feel is a big no-no for boss fights, having minions run around and shit to confound the boss fight in an even more cursory fashion. When Nero fights the Savior statue at the end as more of a 'mini-game' is more entertaining than how Dante had to fight him.
Keep in mind I'm not saying the Mundus fight was great, I'm just saying if you don't know what the fuck to do for a boss fight, at least just have it fleeting in whatever entertaining way can, as opposed to the Savior fight which was not challenging, it was confusing, it dragged on, and it involved too many, "wtf, omg when will this shit end" moments. The Savior fight is probably the reason why I always dread approaching end game DMC4, every time I replay it.
Are you using a PS3 controller? There may be some issues with that. Use a 360 controller if you have one.