You have to shoot chickens of the medieval levels with you bazooka. There are some in the levels, but i'd recommend you to go to the first level (after adquiring the bazooka, of course), shoot the chicken in the very beginning, grab the time trial, die to the goat just forward and redo every step 5 times.
Or you can just do the whole level and search for chickens along the way. Your choice. :D