The main story is, almost universally, very easy. (That said, there are two or three missions that are more difficult for lower-level characters and are better left for later, after first up-levelling your protagonist.

The extra bonus levels, which is not available until you have a character who has completed the main story, is MUCH harder —— most enemies cause their targets to lose one or all of their actions points when they are hit, for instance. I am currently replaying the main campaign so as to play the bonus with a different character, but the main game doesn't elicit much enthusiasm for replay. At least I found it thus.