The challenge comes from the constant onslaught of enemies. I could get to the end of level 3, but that was it until I cheated and enabled invincibility (just to see the rest of the game). I don't know if I'm doing something wrong, but the charge attack seems useless unless you have no power ups when fighting a boss. There is just not enough time to "charge" during the level with all the enemies coming at 'cha.
The game is competent enough but still sits on the C+ or B tier list of STGs for me. Worth a play, but it's not gonna be on my short list of games I'm looking for (mainly due to current asking price)