It's a vampire survivors clone on newgrounds and I didn't notice performance issues, which is impressive.
Things I liked: I was addicted to it longer than I'd care to admit. That's how you know you got the fundamentals down pat. It was fun to play.
Things that could be improved: Once you beat the boss you can keep playing, but you don't really have an incentive to do so. The addictive spell you cast kind of breaks.
If you do a Vampire Survivors type game, people will always be drawing comparisons to it, unfortunately. Applied here, in comparison V.S. has: More levels, more characters to choose from, ways to combine power ups into bigger power ups, so greater ways to customize upgrade paths, and flashier bright casino like graphics that make brain go burrr more. So those are all things that could take this to a higher level. Also multiplayer would be cool but I realize that's a TON more work to pull off.
Also, I know it's not the priority in games like this, but story/lore wise you have an interesting concept about this Vincent butler guy protecting a ghost from evil spirits. I kind of would like to know more, like maybe a page on the menu screen or a character description in character select where you can learn more about who these characters are and why they're here. Again, I get these kind of games storyline takes a backseat, but seems kind of random, butler in happy mask protecting a ghost.
Technical issues: Only real thing I saw was that you can upgrade crit rate beyond 100% which I'm assuming doesn't do anything. Maybe remove that as an option once player hits 100% crit rate?
Also, it seems some of the characters are a bit unbalanced. If you do the upgrade path right I found Winry to be more overpowered than the unlockable characters. You just have to crank up attack speed and add power ups that trigger based on times enemies are damaged, like wisps and lightning. Since her attack goes through enemies and hits multiple, it's easy to trigger those multiple times with that.
Enemy explosions on death as a powerup is potentially overpowered as well. You can wipe out a lot of enemies with a chain reaction of explosions.