Great, keep improving!
The Good:
This is a welcome improvement over your first Zombie Smasher. The custom graphics, the variety of weapons and enemies, the different stages, and the final boss are all breaths of fresh air. Keep improving and you'll have a top notch game in no time!
The In-Need-Of-Improving:
The graphics, while good in a few areas (certain zombies, most of the guns), could really use some improvement in others (the green blob, most of the backgrounds, the health bar, the boss). It didn't feel like you spent as much time on the backgrounds as you did on the guns.
You should definitely add in attacking animations for the enemies (don't just have them move closer to you and magically do damage).
The sounds are also a bit annoying, but bearable. Consider using a free tool like Audacity to make them a bit less painful to the ears (especially the SPAS-12). Or consider using a generator like sfxr to make new noises altogether.
You should definitely add in background music.
I mentioned the variety of weapons and enemies before, and this is definitely an area where you can improve. I feel like the variety offered in this game is a "false" variety. The guns may look different but they all act the same. The enemies may look different but they all act the same. Make guns and enemies have different stats. The boss especially shouldn't be a one-shot kill.
Finally, you'll want to get rid of the "play" option when you right-click on the game. It lets anyone just skip past the level they're on. Just google "flash context menu tutorial" and check out hideBuiltInItems.
The One-Sentence Summary:
This game is a big improvement over the previous game, keep making games, keep improving, and keep loving what you do.
2/5, 5/10