I'm still replaying it. 
There are a couple things to note.
1) There are up to 10 (?) gods you can invoke in any combination, all of which make enemies harder in various ways and compensate by increasing xp/money from killing them. This is the mechanism for increasing the difficulty.
2) You can play through the story again, retaining the stuff you got the first time around, so can can be higher level (i.e. have more passive abilities) and play through early levels using weapons you only get later. It's a little easier of course, so see point 1 and invoke some gods. A major draw of this is just trying different weapons, because they play pretty differently, and you probably didn't master all of them the first time (at least, I sure didn't).
3) There are a few waves-of-monsters areas you can just do repeatedly; they tend to be hard compared to the story IMO. There's also a training ground area for each weapon. I didn't do most of these the first time through.
There are a couple things to note.1) There are up to 10 (?) gods you can invoke in any combination, all of which make enemies harder in various ways and compensate by increasing xp/money from killing them. This is the mechanism for increasing the difficulty.
2) You can play through the story again, retaining the stuff you got the first time around, so can can be higher level (i.e. have more passive abilities) and play through early levels using weapons you only get later. It's a little easier of course, so see point 1 and invoke some gods. A major draw of this is just trying different weapons, because they play pretty differently, and you probably didn't master all of them the first time (at least, I sure didn't).
3) There are a few waves-of-monsters areas you can just do repeatedly; they tend to be hard compared to the story IMO. There's also a training ground area for each weapon. I didn't do most of these the first time through.
