An addition on MajesticPanda's solution:
For the boss level to start you'll first have to play level 4 (duh
...). After finishing level 4 the game will not save but will continue with the boss level. (If you fail the boss level you will have to restart by playing level 4 again.)
To get through level 4 just do the same as with the prior levels. Use the powerups the game provides (you have to buy them from the store first, otherwise the game won't drop them ingame), memorize the paths you take, and don't focus too much on clearing the level with 100%.
The boss level itself is pretty easy. The only thing you have to do is avoid the boxes being thrown of the cart. Don't try to jump over them as jumping takes up to much time hindering you in avoiding the next box. Just weave around each of them, you have plenty of time for that. As soon as you see a red cirkel on screen: shoot!. The bottle of booze in the boss's hand will explode and you'll have one shot less to take :)
This boss takes about 12 shots before you've won (sorry, won this one first time trying so I didn't count the number of shots fired).
Good luck and cheers