Hint: Unlock Killer 8
Beat the game to unlock Killer 8 mode. This mode has enemy critical spots that are invisible, clue and map icons hidden on the map page, and enemy explosions do 5x the normal amount of damage
Hint: Unlock Hopper 7
Beat Killer 8 mode to unlock Hopper 7 mode.
Hint: Thick Blood & Serum
You can't carry thick blood from one stage to the next, but you can with serum. Basically you'll want o collect as much thick blood as possible in a stage and break the machine before moving on to the next level, since it won't work if you revisit a stage.