Nice blend of old and new
First off this game is really well done, if not for some tweaking here and there it would be amazing. I had the same complaint about the difficulty until I unlocked a few moves and some of the cheaper quick upgrades that really help.
That being said I believe it was still very tough even on easy at first. On easy bosses should have less life and I think the extra enemies during the boss battles should be taken out to assist in the learning process of the bosses moves. A majority of the gripes on here are valid, Fat Boss is too tough for early boss, and 2nd boss is kinda easy in comparison.
Few Tips that really helped me through the game: Learn the swoop kick and rising dragon early to help destroy trash mobs quickly. The first few health and SP reduction power ups help a lot, along with the gun cool down and str increases. The first boss just spam swoop kick, regular attacks don't do enough dmg unless he falls over. Once you get past him the next levels net you much more exp. The 2nd boss just keep your distance and jump over him when he attempts a combo, and attack from behind with your own, or again swoop kick is great from behind as well. 3rd robot boss is frustrating, but if you make use of your block and your evade roll to get behind him often it sets him up for a few hits. Use berserk when he flashes blue and swoop kick him as fast as possible to finish him quickly before he takes you out and he shouldn't be too much trouble.
And lastly if you get comfortable killing a boss (2nd one for me), try to finish him just using your gun to chain up a fairly large combo, like DMC a bigger combo when enemies die nets you much more exp. Good luck I hope this helps through the frustrations that may make some stop liking this game as much as they should.