I'm glad you liked it! For the boss, it's like a big mech (think N.Gin from Crash Bandicoot 2/3, if you're familiar). Try to destroy its cannons. If a heart (one of your bullets) plays a little explosion on impact, that means it did damage.
Also, I uploaded some bugfixes a little while ago, so if you downloaded before that, I recommend redownloading it.