ALL WEAPONS, FULL ARMOR, HEALTH AND AMMO
Press the back button to access the objectives, then press: up, up, down, down, left, right, left, right, B, A

FULL AMMO
Press the back button to access the objectives, then press: B, A, X, Y, left, right, left

FULL HEALTH
Press the back button to access the objectives, then press: B, A, B, A, up, up, down, X

To get all the 'complete a level with a certain gun' acheivments easily there is a whole level which is just an elevator ride(dont worry it's short).its in one of the 3 towers. Save before you go on the elevator and reload after and swich weapons

Red,green,blue,yellow,left,right,left