All you need is the triple damage potion for the bosses. Youcould skip the isles to go directly to the bosses. I suggest that isles should be locked until you kill all enemies. I just saved max armor/weapon with the +25% atk song and I was able to kill all bosses easily.
I like the easter eggs like mournshadow sword. :3. I would suggest enemies that are weak to magic or weak to physical so that we could alternate between the two characters. :)
Losing speed is faster than gaining speed .-. Grinding money is way too slow. The effects, game-play, and graphics are great but survival mode isn't that worth to go through. 3/5 for me.
It's just annoying that all your hardwork can screw up with just a single miss. Done illuminator badge with assassin main+assassin aggressive+stupid vendrux on defensive at lvl18 all.
For the boss, spam the chaotic trance or dragonfall. Weakness: Minoutaur-Freeze and Soul collector, Dragon-Freeze, Knight-Stun, and the Violet Goblin-Fire Sigil/Burn.
You just need to summon pyro tengu, striker pengu, and fire sigil at front. It makes the wave easier. Also max the burn and freeze effect. Max the arcane surge too and get the kinetic charm on merchant. This works perfectly fine for late game. If you are in a pinch, storm pengu would be your ally at ground. Freeze amplifies the next different elemental damage it gets and also got cold collector for it.