Zerglish....
bwahahahaha.
ahem
Okay I am better.
Seriously though, I'm not sure if you were intentionally trying to go with that "English isn't my native language" feel on the back text, but it is the only thing preventing me from wanting to take this bad boy to kinko's. :P
Might I suggest "Metal Slug 3: More action than you can shake a gun at! Go Go Go!!!"
It should be noted that one should really not end a sentence with "at", but since I assume you were attempting to use the "shake a stick at" platitude it is okay.
The inherent problems with your existing text is the misspelling of "with" (wiht) and the fact that saying that it is "Metal Slug 3 with a lot more" indicates that somehow this game is more than Metal Slug 3...which of course makes no sense.
Hope this makes some sense...
RevQ