To do this it is very simple and easy.

All you do is manipulate the settings like skulls, difficulties, weapon damage, ammo and invulnerability and you can easily get the Legendary achievements by simply switching them around.

Legendary with all the skulls on makes the game multiply your scores and when you change the time limit and set it to unlimited, you can gain credits depending on how long you play.

I played for thirty-three minutes and fourty-two seconds and got through the first set.