First you must unlock the one shot one kill cheat.You get this cheat by killing the end boss on time trial.
You may have found out this cheat does not work on the hard level, this is how you get it to work.
You have to enter the cheat in time trial then quit time trial. Then load your game that's on the hard drive and the cheat will work.
This cheat will work on some of the bosses but not all of them.