Hmm... I feel I can't rate this game, 'cause I have'nt played it multiplayer. Since my campus network is very restrictive, I'm using a VPN right now. And assuming that you're from the US, I'm halfway around the world(India): timezone issues. So, I don't think I can make multiplayer work. And I feel that, since I can't play it in the intended way, I can't rate this game.
Anyways, I can tell you my thoughts on the singleplayer experience. It was quite rough. I could never reach the boss fight you mentioned.
Those Skull enemies! Is it possible to kill them without losing a ton of health? Also, I feel the enemies should be slower than the player, rather than accelerating and then decellerating to get to them. Maybe make the enemy summoner's els' easier to kill too? He just keeps spamming those.
Love the aesthetic, though! Great artwork! Also, the very fact that you even tried a multiplayer game is great enough!