You can run the Jar directly, or have the batch file and the jar in the same directory and run the batch file. You also need Java installed.
Viewing post in Havok jam comments
I tried your game.
In general, I would agree with the others in the comments, but you did this from scratch in less than a month. That's impressive. The biggest change I'd advise is making the bullets bigger. May I ask why you chose to make your own engine, rather than using an existing one?
Though, for the sake of fairness, I'll rate your game the same way I would others.