What I consider to be the main problematic area in Hard mode - both the level and the boss. It is very easy to go flying off the screen from one of Stings jump attacks due to there being no means of interrupting it, so be sure to press up & attack simultaneously to destroy flying drones.
Also, there is only one door at the very top of the first area to go through in order to save your kill/collection progress, so start the level by just focusing on working your way up to the door. Exit and reenter the first area and steadily begin killing off enemies and collecting gold but be careful because some enemies may fall off the rocket too and it will not count towards the percentage progress.
The boss requires a very specific and long winded strategy to defeat - stay against and attack the right hand corner of the room repeatedly to avoid damage and wait to attack the eye ball quickly every time it falls.