To those people that click "start builder" or "select gametype" and notice the "you have to beat Decem first!" description(I already beat it so i cant type the exact text sorry), I guess you looked around online and didnt connect the boxes in your noggin(your head you goof), hes obviously an enemy in the survival mode. He is pretty far in the waves if you want to fight him. So climb the waves! Fight hard! Learn combos! Check out the shop and mix in new skill sets! Utilize your items and skills. Also, I'd like to give a tip, that you can switch up items during combat. I have specific items for certain enemies which maximizes my effieciency in fighting. Lastly, when fighting Decem, figure out how he fights and then figure out how to counteract his fighting stylebecause you do not want to die and reclimbthat whole ladder again. Thats all I can say, Good luck fellow fighters! Note: It took me about uhh 5 times until I figured out how to beat him and I was playing on Turbo mode because why the hell not?. It will be frustrating but you'll get it if your determined enough.
Regarding the "item switching" thing, we actually do plan on having a loadout system and a button to switch item/movesets mid-combo too at some point - so that's a pretty good strategy to have.
Decem is the only AI in the demo that can do everything the player can, except for aerial grapples. So you have to employ everything you've learned fighting Fliegers and Reapers when you go against him. One on one fighting usually requires a lot of weaving and aerial recoveries, so practice those inputs in the practice mode if you need to.