From the Main Menu, go to Options, then Password. Type in the following....
W83M 7H20 BDY0 N9VB
This spawns you on the last level on easy with max weapons and stuff. I found the easiest way to get this was by just rushing the boss, otherwise her homing rockets killed me. Here's the method I used.
until you get to the last weapon. This weapon is called the Unmaker.
2. Collect the Invulnerability Orb (The green one on the right)
3. Go to the door and open it by pressing
4. Run to the centre. This makes all the enemies spawn. Now you need to stop the enemies as fast as possible so that the boss spawns. You do this by.....
5. Press the 3 switches around the outside (with
). Each switch closes the door where the enemies spawned. Some will always get out., but if you do this, you should have some invulnerability left.
6. Kill the remaining enemies as fast as possible so that the boss spawns in the centre.
7. Run right up to the boss in the centre and keep hitting her until she dies. If you did step 5 and 6 quick enough, you should have lots of ammo/armour/health left to be able to do this.
Here's a video of me doing it.
Hope this helps.