Hard mode isn't different at all from easy only thing i noticed is you will lose the auto aim with your gun and bosses has more heath.
Here is a full walkthrough of the game incase you are stuck on certain spots and that will happen most likely.
Update: Intestinal Distress boss can be a pain but i found a glitch to take him out easily, go to the far left screen and stay there the boss will jump to left of you and will damage you one time and he will get stuck behind the rocks from your same position aim on top left and start shooting toward him you will hear his voice when he is taking damage keep doing that and claim your easy win.
Update 2: Once you reach the second part of Buttville just lose all your lifes and the achievement will pop, A nice glitch will save you the troubles of the last level.