Tetrahedron Dash 
#1 Factory Dash simp
You think those were hard? Try battling Thunderhead, while underleveled, maining spells, and only having 30 regular/small heal potions. I died so many times, but eventually beat Thunderhead finally. The trick for me was to use Magic Beam II to take out almost all of the turrets, and then use Black Sun II to start attacking Thunderhead. The orb things that come in when Thunderheads HP starts going down near the middle are very dangerous when there are lots of them near the end of the battle. You MUST teleport away from them or the massive contact damage will make you take even more damage, and at this point, you are probably taking so much damage that you can't afford to regen health anymore (besides potions). At that point, its a matter of whittling Thunderheads HP down with Black Sun II and take it out before you are the one who gets taken out.
The Zebra boss, Mchavi/Necromancer was the boss that gave me the most hell, but eventually I beat her. Magical Shield + Distortion Field + Moonblade = OP. Just make sure you have lots of potions to heal with, lots of magic potions, especially lots of potions of purifying. Thunderhead though is hard for different reasons, one being obviously that it is the final boss, but two being that it is a really intense and difficult bullet hell.
The Zebra boss, Mchavi/Necromancer was the boss that gave me the most hell, but eventually I beat her. Magical Shield + Distortion Field + Moonblade = OP. Just make sure you have lots of potions to heal with, lots of magic potions, especially lots of potions of purifying. Thunderhead though is hard for different reasons, one being obviously that it is the final boss, but two being that it is a really intense and difficult bullet hell.