The game is hard after the 1st two levels, even on casual difficulty.
Dying over and over again doensnt reward you with any credits, so you either get good, or start from scratch or something to able to beat the level, or at least get to a checkpoint.
No one knows where these checkpoints are, and in the 1st level, if you die to the boss like i did, you start from the middle of the level(were the checkpoint was) instead of the boss fight.
Its still better than starting from the beginning, i agree, but the main issue is that it get way to hard withouh earning credits to upgrade your weapons.

The bosses on the 1st chapter are relatively easy.

There are 4 chapters in total, with 6 levels each i believe.