Open the console with F12 and then enter the following command:
var r = cr_getC2Runtime();
Then enter the following commands one-by-one. After each command except the final (lvl15) one, enter the game from the "Continue" button in the main menu, pause with F5, and press Esc to return to the menu:
r.all_global_vars[13].data="lvl4"
r.all_global_vars[13].data="lvl6"
r.all_global_vars[13].data="lvl11"
r.all_global_vars[13].data="lvl15"
You should now be in the final level. Enter this command to give youself the GRINDER achievement:
r.all_global_vars[30].data=1000;
Finally, complete the level with the default character (killing as many enemies as possible to make life easier), then press Tab to swap to the other character and complete the level with them as well.
Cheese strategy provided by GENiEBEN. 
