The difficulty is balanced between the single player and co-op game, 'cos both use the same campgain, what you fight in single player is also what you fight in 4 player co-op.
Its not a walk in the park, but once you get the hang of it you don't meet anything that feels stupidly hard.
btw, normal attacks only knock you back if they hit you while your wearing a personal shield.
Its a fun game to have a bash at now and then, "accidenty" frying Sax's brains is always a laugh.
ps. i've you've got it too we can all load it up sometime, will keep an eye out for when we're all on and free