That rusty silent-hilly look is so cool!
Bosses are creative and fun to figure out.
With only 3 hp (vs 105 of the first boss) and such a huge difference in the speed between me and bosses, the only viable strategy was to throw the dianthus away and simply focus on running from projectiles while praying that the dianthus would randomly knock the boss out.
Placing most of the controls on the mouse is a great idea for this game, it helps to move precisely together with making faster "dashes" (well, as fast as the speed limit allows, which sometimes felt like an unfair drag). Maybe you could also map X onto M1 to completely eliminate the need for a keyboard, and it might also play great on a touch-screen (with a couple of on-screen buttons).