In my first couple of tries I couldn't get the achievement to unlock. Turned out I was adding health (pressing Y), and with each press, my score got divided in half.
My own method worked really fast:
- Use any character
- in level 1 at the start, go down and get the key (don't exit!). Shoot your way through the ghosts
- Open the door on the right and move quickly past the ghosts and spawners to the Level 4 exit (don't worry to much about taking damage)
- You'll then find yourself in level 4. Get the key (and potion if you want), open the door (right-lower corner) and shoot yourself up through the enemies, killing the spawner as well. You can use the potion if you like.
- at the top, turn left (only way you can go), killing the enemies and spawners.
- At the end, go down (kill the spawner near the top).
- Now stop just below the middle of the corridor. At the bottom, just around the corner, is a spawner which spawns ghosts.
- Since you can't hit the spawner, you can just keep shooting the ghosts that appear and come arround the corner.
- If you have around 450-500 health left, you;ll be fine. (I think I even made it with less then 400). You'll reach 8000 before your health drops to zero
- If you don't move and just keep shooting, the walls will become exits after awhile and you can shoot through them. When this happens, you can also shoot some other enemies at the bottom (in another corridor) which will help your score getting higher even more quickly.
- after you reach 8000 the achievement will unlock in-game so you'll know you succeeded. You can just let yourself die or quit and do the same thing for the oter three characters. I got all 4 achievements within 10-15 minutes.
If, by accident, you've taken too much damage to reach 8000 with one life, you can add 700 health points by pressing Y. You just have to get a score of 16000+ instead of 8000. The achievement will unlock in-game, so you'll know when you're there.
Good Luck!