I recently realized that Sheik's DACUS is much easier than I thought. I was just inputting other aspects that I hadn't originally considered incorrectly. Keep in mind that I have the Z-button set to attack (so much easier, I just have to get used to Y as grab), but it should work the same for normal configuration users.
I'd always thought that my biggest problem with the DACUS was my incorrect timing of her Usmash during the DA. When I messed up, I'd either end up doing a DA or Fsmash, and figured that I was just inputting the C-stick tap too late or too early, respectively, because the frame opening was way too tight. Now I realize that this was only half right, and that although it is still the tightest opening for all the DACUS's in the game, it is not nearly as bad/hard as I originally thought. In fact, as long as you don't input the Usmash too late, inputting the Usmash anytime after inputting the DA correctly will ALWAYS produce a successful DACUS, if you are doing it right; eg. there is no such thing as "too early" as long as you've tapped the Cstick down after a dash. If this doesn't seem true to you, then you are probably making the same mistake I was, so keep reading.
If you are coming out of an attempted DACUS DAing instead of performing the intended DACUS, then you are in fact hitting up+Z/A too late. But if you are doing an Fsmash, then you are inputing your Usmash wrong. It sounds ridiculous, right? It seems like such an easy thing to do. But what I discovered was that when I took out the C-stick tap from the input sequence, (Control stick tap forward [dash] > up+Z [Usmash]) I was actually doing an Fsmash instead of a hyphened Usmash. When I saw this, I just though; "... No way." I practiced for a few seconds to do the hyphened Usmash correctly, kept the sequence in my mind, and then just added the C-stick tap in-between the dash and Usmash inputs. This resulted in a flawless DACUS which I now do successfully almost every single time. Turns out I was so convinced that the timing was ridiculously incredibly tight, that I was rushing my Usmash in. The result was that I was actually hitting Z (attack) before my control stick had the chance to move from "forward" position to "up". Forward + Attack = Fsmash =/= Usmash = Up + Attack.
Long story short: IF YOU CAN DO A HYPHENED UP-SMASH USING THE CONTROL STICK TO DASH AND UP + WHATEVER BUTTON YOU HAVE SET ON ATTACK TO UP-SMASH, THEN YOU CAN DACUS BY JUST ADDING A DOWN-TAP ON THE C-STICK IN BETWEEN.
DACUS. IS. NO LONGER. HARD!