I was glad to see "v1.2" under the title, as this game definitely strikes me as a work in progress. If that's not the case, you can feel free to ignore my suggestions, but that would make me sad as I think the game has promise.
*Experience = a dozen or so play-throughs, max score of 1200-something
Difficulty: The game is hard, sure, but its difficulty lies more in luck than skill. Any given run is a crapshoot as to whether you will find enough upgrades to be able to kill the turrets before they kill you (since they begin firing so soon after they appear on the screen). I had about 1/5 chance of that - once I got up to 600 without running into any upgrades. The curve needs to be fixed a bit so that most of the time the player has a chance of competing with the turrets. This could be done with more upgrades earlier, waiting to throw turrets at the player until a bit later, or my below suggestion of having a variety of upgrades. In addition, even on my longest run I had no more than 3 silver and 1 gold medal - making those more manageable might not be a bad idea.
Variety: The difficulty could be helped a good deal by some additional upgrades. Allowing the possibility of tank repair would do a lot to balance out having to wait for your rate of fire to increase. On the fancier side, temporary weapon boosts (spread shot, added damage, etc.) could do a lot to liven up the action.
-If you choose to replay the game, the medal counters disappear and you occasionally start with damage.
-Some turrets don't fire any bullets.
-The Sniper medal doesn't appear on NG when you achieve it in-game.
Side note: It seems like work was put into the sound design - more than I was expecting from an 8-bit game. Nice job!
Anyway, I hope this was helpful and I look forward to playing a future version of the game. All the best.