I need help with Sheik, I have an incredibly hard time actually killing with Sheik thanks to no reliable kill option (at least as far as I can tell) and I have a hard time knowing when to grab so I can even reliably get some good damage which will allow me to get the character to 150% without losing a stock. If someone could help me mitigate these issues, I would greatly appreciate it. :3
Try getting a kill before your opponent reaches like 140% or above, because then it's a lot more difficult to kill since none of her set ups would work at such high percentages. However, if you do reach 140% and above on your opponent, what I normally do is camp with needles and try to either kill with a Fair near ledge or an up air, or in extreeeme situations camp and play super safe until he's in KO range of ftilt (~160-170% with rage iirc? not sure)
At 100-130% Sheik can have some combos that could lead into deaths; weak nair to bouncing fish (good on paper but a bit harder to execute in practice imo). One thing to note about this is that even if you hit opponents with the hitbox behind Sheik and let's say, you're facing the right when you do fall down nair and opponent is knocked back to the left, you can still do a reverse bouncing fish and hit them.
Bair is a very good kill move so if you get a read with that, then props.
Something I've been doing on a whim instinctively when playing Sheik is ftilt into uair which actually kills with rage at ~110%-120% and I'm not sure if its a true combo or not, but it's definitely a very reliable kill set up that I've been using. I definitely need to lab that soon.
Gimping is a cliche answer but it really works best; try catching characters like (Dr) Mario, Falcon, Marcina, Ganon (lol), and all the characters without ridiculously long recoveries or warps (such as rosalina, pit, zelda) on their second jump, throw some hitboxes, even if weak, like weak nair or bair, and they'll really struggle to get back. Against characters like Rosalina or Pit(too), throw a nair low off-stage when you think they're about to Up B and it really harasses them quite a bit, Rosalina moreso than Pit from my experience.
And finally, my personal favourite and probably the fan favourite too, full hop needles into bouncing fish. If you don't know how to do this and you use a gamecube controller, I highly recommend using customized controls because otherwise this is very difficult to pull off. I set Z to jump specifically for this combo, but L can also work. What you would do is release needles the instance you full hop or otherwise you won't have that few frames window to initiate bouncing fish before landing and you'll suffer from lag. This works best as a gimping tech from my experience but you can land unexpecting opponents on-stage too, although I find that to be unlikely.
Also, vertical needles are good when gimping in general because it harasses so many people who try to recover low.