Rare Replay

This achievement, requiring you to defeat a cumulative total of 1,000 enemies across all playthroughs of the game, is best done separately from your main playthrough, as that playthrough is made much less frustrating with the use of the 'no enemies' cheat. To unlock this achievement, first turn off this cheat, then find a screen with a large number of enemies - the screen that's at the lowest level of the Underwurlde will work fine, but plenty of other areas will also work. You'll find that the enemies will respawn continuously, which in the case of this achievement is very helpful.

Now position Sabreman at one edge of the screen and hold down the attack button (with a weapon equipped, of course) - you should find that you're able to defeat enemies fairly consistently like this until the achievement eventually unlocks. You may even be able to leave your console unattended if you have a method of keeping the button pressed, especially as enemies can't actually kill Sabreman, but keep an eye on progress in case an enemy manages to collide with Sabreman and disrupt his position. 

Note that you can't use the rewind feature to work towards reaching this cumulative total.