The Mirror boss: You know, that stupid thing, that thing will be getting some changes. I made it and even I dont really like it much. It's got a whole pile of problems right now. The actual arena room it's in though probably isnt changing. But the boss will. Just.... ugh. It practically is just one giant design flaw right now.
Now, as for where to put feedback, well, there's a couple of ways to deal with that.
For the most part, I think that most of those that are working on the game dont really have time to check these forums all that much, aside from myself. I usually will then relay important things to them when necessary, but a better way to make sure they see really important stuff is to put it here:
https://arcengames.com/mantisbt/my_view_page.php I know that looks like a site purely for bug reporting, and that's it's central purpose, but it's a really great place to put down suggestions or list balance issues or things like that. Mantis basically guarantees that everyone will see it, and that they dont have to worry about forgetting about it or anything.
But making topics on the forums can be good too, with discussion and all. In particular though, if you have feedback about bosses, enemies, or things that are in any way related to them, bullet patterns, or many parts of combat, feel free to simply leave those bits here, since bosses/enemies/patterns and such are all in my realm, and it's super easy for me to make any necessary fixes to those specific things.
Also you have a good point about the range on special weapons. That's not something I personally can fix directly (not my area of design) but now that you've mentioned it... that might be an issue that could do with some looking at.