Solid game as I've been playing it so far, but i do have a few notes about it.
1. The gun feels like trash, like honestly. I'm level 15 with it, and while it's on hard mode at level 2 I'd expect everything to be paper at that point. It really feels like it does nothing while hard mode is "Hard" is the multiplier so up there that this sort of thing doesn't change anything? Especially considering the cost involved with upgrading in this area lvl 16 alone is 101k which takes farming a good bit of it for one level only to go up while feeling like I've gained nothing. So either gun should be buffed or the cost should be cut for this one.
2. The mouse controls are extremely non-time responsive. They lag a lot compared to the key controls which makes things kinda a pain if I feel like I want to use the mouse. Which begs the question why not just make it a build your own control scheme for the last option? It'd make things simpler and allows people to build a more customized and player friendly control scheme.(Have a VERY tiny up arrow on my computer keyboard making grenades impossible half the time.)
3. Hard mode and shields: Maybe just maybe it's my luck isn't high enough and this is why I'm not seeing them. I can get lives not dropping but shields not at all? Not even a rare shot just no shields period?
4. Stomp level up, in truth I haven't touched it and not really sure I will because there isn't a lot of point to the major risk that comes with it. Perhaps somewhere along the chain(10+ increments) you could add a saver function to it. Where when you go to stomp but miss you wont take fall damage one time per increment of level. Gives people a merit to want to try using it but also gives a net which only comes with more extended play of the game.