Hi,
The game doesn't seem to ahve any levels per say which is fine, but the enemies get stronger and faster.
And now, some bugs and suggetsions, let's start with the bugs.
The taser seems to work nicely, but there doesn't seem to be any taser upgrades in the shop. Also, the pusher itsme seemed to do nothing in the shop. I can't upgrade the pusher range so it ets to the point whre it gets very hard indeed to keep up with the enemies as he just goes coming right towards you.
Now, some suggestions, more specifically some unlockable modes, I realize this could be a bit tricky to implement.
You could select a mode from the main menu as a setting after start game, perhaps. Modes get unlocked as ou gain kills. I hope I have a nice balance here, at 500 I chose three modes, you'll see why. I've done what a good production could do, allow things to get tense then lesssen up a bit then get even more tense then very tense at the end. Here goes.

0 kills two modes:
1. Classic: the way the game works now, with all saves in play.
2. Summoner: multiple enemies are summoned. Instead of enemies getting stronger each wave increases the number of enemies, chicken coop style. All saves in play.

100 kills, 2 modes:
3. Greater summoner: employs both wave methods. Enemies get stronger and more sporn. All saves in play.
4. Endurance: Start out with 100 health, 100 gun ammoe, and 200 machine gun ammo. Classic enemies, but disable the shop. Allow health, gun range, and gun damage upgrades as items, small packs of ammo.

200 kills, 2 modes:
5. Summoned endurance: Endurance but with summoner waves.
6. Summoned Greater endurance: Endurance but with both types of waves. Disable items as well for an even greater challenge.

300 kills, two modes:
7. Messing around: classic enemies, but each one has different health. Sometimes it goes up sometimes down, and sometimes it's a one hit kill.
8. Messed up completely: messing around, but also add trick items so it might tell you it's a gun upgrade but it's actually a machine gun upgrade, and a super health boost is just a normal boost, and an enemy speed up is an enemy slow down. Dont completely change the items so it's totally random though, see why below.

400 kills, two modes:
9. World of  Roulette: Here's where it might tell you anything, and anything can happen, so a health upgrade could be a machine gun upgrade, or an ammo pack could be a health pack or even a water blaster change. Enemies have random stats.
10. Poisoned: classic enemies, but you lose some health every few seconds. Allow items and shop.

500 kills, three modes:
11. Bullets of Poison: same as poison but only happens if you get hit, and the more you get hit the more poisoned you become.
12. Warped: every now and then your arrow keys reverse. Classic enemies, allow items.
13. Disorientated: every now and then your stereo field reverses. Classic enemies, allow items.
Note: make sure both these modes unlock before the next one because the next mode is tricky.

600 kills, two modes:
14. Insane Mind: both warped and disorientated. Classic enemies. Allow items.
15. Spaced Out: Every odd challenge. Item Roulette, random enemies, warped, and disorientated.

Time for the hardest tiers, and this time it takes longer to get new modes.
700 kills, one mode:
16. Asylum: make stuff happen twice as fast, so enemies sporn every two seconds, summoner waves and stronger waves too, same with items. Imagine a field with six enemies and a ton of items around.
800 kills, one mode:
Insane Asylum: Warped, Disorientated and Asylum
900 kills unlocks two modes:
Depths of Insanity: Warped, disorientated, greater summoner, Asylum.
Mark of Death: same as depths, but with bullets of poison as well.
1000 kills, final mode:
The Living Hell: Warped, Disorientated, Asylum, Poison and Bullets of Poison.

Kills can be racked up in any mode to unlock, as I believe trying to gain 800 to 900 in any of the asylum modes could be way too difficult.
I know this could be tricky to implement, as I say, but I think it'll give people another reason to gain kills, and a goal so to speak. This way you're not just unlocking minigames but also game variations.