i use the y button instead of x lol. but for me, i tried this 1/4th speed at first but then realized that this could hurt me with timing. but heres what i did to waveshine:
1. practice WDing. theres no point in waveshining if u cant WD yet. just get really good at this (try to WD around the stage for lengths of time without messing up).
2. practice doing a single shine and WD out of that as quickly as possible. slowly just add more shines into the mix until u can comfortably shine > instant WD > shine > instant WD and repeat.
3. practice on someone marths weight or heavier.
the biggest thing about getting a perfect waveshine is getting the WD length the max it can be. you have to alternate between the control stick angled slightly below the horizontal (as in closer to the side b position, as this will give u the best angle for a long WD) and having it more vertical (so that u can do the shine after the WD).
For multi shines, definitely practice with falco first, then wolf, then fox. fox has the strictest timing, falco has it the most lenient, and wolf is in between. you can shine then jump then shine again, and just try to slowly get the timing where u shine closer and closer until the ground, until eventually you are canceling the jump squat and not even leaving the ground (this is the ideal multishine). theres 3 different methods i would recommend:
1.i just hit down b, then instantly hit y/x and slide right back down to b. you can keep holding down on the control stick. you are at no risk of doing a different move with this method, its just about how fast u can move ur thumb.
2.alternatively, if u use tap jump, u can avoid the whole sliding ur thumb to the jump button and what not, and just flick the control stick repeatedly. with this method u just have to be careful when u hit B, as there is a chance u could do an up b or neutral B instead.
3. a third method that i find useful is setting the L button to jump, and just hitting down b immediately after hitting L. you can keep the control stick held down (no risk of doing a different move) and just time ur button presses quick enough. the only disadvantage to this method is that sometimes the shoulder buttons' bulkiness make it hard to time the press. i often just jump and not even shine because i pressed B right when i pressed L, so it misses the B input, but this is not as risky as an up or something.