you can kill final boss once you unlock machine gun. rocket jump to the floating lift above the mach gun. to the left is the lightsaber, and to the right, with another good rocket jump you can get to a chest. beyond that is boss room.
also, easy way to kill boss; jump on his head and lightsaber him.
cacomon with lightsiber: can use other weapons until you clear to him. rocket jump on his platform above him. you want to be in the center of it. every couple passes he will be in range to swing at. the hardest part is getting it set up. killing him is a joke, but it takes forever...
you cannot collect items from chests you sold. save your socketed armor, you will need it after level 18 if you plan on getting achievements. very strange bow + speed gems + damage gems = gatling gun. so fun!
i've heard that the average person has a vocabulary of about a few thousand words. there are hundreds of upgrades in this game with unique names. it doesn't take long for the words used here to step outside your vocab. do you know who bishamon is? or what an infilade is? i challenge you to go look up some of these things.
the items from the shady merchant do not sell for more than you pay. they have higher value than his price, but items sold to vendor are less than their listed tooltip value. the shady merchant has some nice stuff depending on what rng you have had, but you are not going to make a profit from him
yes, emporium badge is worth it. it pays for itself quickly from all the stuff you dont have to discard. better yet, you can check the shop for upgrades and enchants more frequently