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Cool. You have a solid base for a potentially good shooter game here. Let's start with a few important notes:

1) There doesn't currently appear to be a difference between the various "enemy" shapes, which can be a bit confusing. Players are used to different sprites/enemies indicating a mechanical difference they need to react to. (Eg: differing attack types, differing health/attack strengths, differing speeds, etc.) Each of these differences should require the player to employ different attack/defense strategies in order to keep the game feeling varied and fun.

If these differences already exist in the game, it wasn't clear/obvious; so consider how else you can communicate this information to the player and/or make these differences more exaggerated.

If they don't already exist, consider adding differences to each enemy so it forces your player to change up their tactics. Also, don't add differences at random, consider how various differences will practically impact a player's strategy. Then only add ones that make for fun and interesting strategies. Trade-offs are great for this! (Eg: Small & fast enemies are easier to kill, but more likely to damage you; while big & slow enemies take longer but could do more damage to you. So the player has to consider which to go after first.)

2) It doesn't feel like there is an explicit goal for the player to accomplish (other than avoid failing/dying). Think of adding some kind of clear goal to accomplish, such as: survive a "wave" of enemies (then rest and heal-up for the next wave), traverse an area to get to a goal, protect another object(s) in the area, collect something, etc.

3) There is very weak "feedback" to players for what the actions they are taking (Good or Bad) in the game. This feedback is sometimes called "game feel" or "game juice". I'll link to some resources below for more information on this.

Once you have these 3 items nailed down, not only will you have a really satisfying game experience but designing a boss fight should be easier because the boss will simply be a ramped-up challenge of the existing strategies/mechanics already in play. 

And if nothing else comes to mind: make your boss HUGE, let them give/take LOTS of damage, and have them release very small/fast minions you have to get through at the same time.

(Also, the quit button on the pause menu isn't currently working.)

I hope this helps! Please feel free to ask any follow-up questions.

Here are a few resources on "Game Feel":

What is Game Feel:

How to add Juice to any Game:

Making pressing a button "Fun":

A More in-depth and academic look at game feel: