This is nothing more and nothing less than a cheap zombie-kill game with a lot of bugs and bad design decisions.
1. No tutorial
I don't need much of one. One little screen saying 'Click to shoot, M to mute, P or space to pause, R to reload' would have been enough. You can't assume that these are known (in a lot of these games, for instance, space is reload).
2. Lack of committment to the style
I played this game because it had Mario and Sonic on it. The problem is, while the players and the enemies fit the theme, the most important part of a zombie shooter - the guns - don't. Forget pistols and grenade launchers and colour it up with something at least remotely based on those characters. Fireballs, maybe, or a ring-gun. I don't know. Be creative. Also, forget military consistency; Mario does not belong in a green helmet.
3. Strange enemy design
There are certain times when convention is a good thing. When I'm introduced in level two to flying enemies and goombas, hours of playing these games told me that they should be either faster and weaker (fliers) or slower and tougher (goombas). Instead, they were both faster and tougher. You're not supposed to upgrade both that early.
4. No choice
I like Mario, but I like Sonic more. Could you not have introduced an option where the player could switch the roles of the two? It couldn't have been that hard; you already had the shooting animations.
5. Odd level progression
There seemed to be no pattern to how long the waves lasted. Sometimes they were ten seconds, others over a minute, with no sense of climbing scale as the game went on.
1. Useless Goombas
When Goombas reached my wall, they seemed to do no damage.
2. Headshot variety
Sometimes when I got a headshot, I'd do bonus damage. Other times, I didn't. Which is it?
3. M for music?
Pressing the M button toggles the display in the corner that says 'sound on/off' but it does not actually toggle the music. I had to click the button manually to get the sound to shut off.
4. Oh, so you want quiet?
At the beginning of each round, the music came back, despite the sound being set to off. This required me to click the off button twice to get it to shut up again.
There could be a decent game here, but it's a lot of work away.