Really good game, I love the retro feel to it. I love the art style and music. I can't stop thinking of the NES Contra while playing this. The difficulty is usually perfect. But there are some flaws for me.
- Sometimes the games alignment and placements are awkward. Like there are spots where an enemy is above you and you are on a slightly lower platform. But you are too low where you can't hit that enemy while standing, but you are high enough that the enemy shoots a straight shot and it hits you in the head (like on the beginning of the level 5) forcing you to jump and perfectly time your shots unless you got the flamethrower or an explosive weapon.
- The shotgun is alright, but with it only shooting 3 far spread out bullets with a pretty slow fire rate, It is a really situational weapon. I know it shouldn't be an OP Contra shotgun, but maybe 1 buff of an increased rate of fire, extra pellets shot (4-6 total), or bigger bullets for a bigger hitbox could make the shotgun more useful (maybe my suggestion at the bottom could implement these upgrades).
- It sometimes is annoying when an enemy that was killed respawns instantly if it's spawn-point was barely off-screened completely. Maybe require it to be a bit more off-screen before it can re-spawn.
- When you die and drop your weapon upgrade. It is impossible to get it back on time because it despawns in about 3-5 seconds. But since enemies and boxes re-spawn, you got to get through those before you can reach your already despawned upgrade.
- In the final boss room, the moving rail platforms near the spring platforms are really janky to jump onto, you need to be at the very peak of your jump, otherwise you will start getting on the platform, but fall right through it. This makes it easy to take a lot of damage when the level 5 robot boss corners you.
- The robot boss is TOO unpredictable. I have absolutely no idea what he will do next. I do want some unpredictability, but sometimes i always am nervous if he will jump ahead of me even when he is not far behind me, so it always lands a hit on me.
But still great game, I love it. It is not easy making a great run and gun game like you did. I hope my review helps you with your game.
Also I think a cool idea to maybe implement is a mini upgrade system to weapons (if you played the arcade version of Super Contra you will know what I mean), where if you pick up a weapon that is the same one you have equipped, it gives a small upgrade. Like increasing the fire rate some, increasing bullet travel distance and speed, increase bullet count limit (increased pellets for shotty, extra bullets for burst, longer laser, and bigger explosion for explosives), or even wall penetration for some (or bigger hitboxes if they already penetrate walls).