Honestly like you can take the advice I got from Chok that has been helping and going against level 9 CPU's that are disadvantage matches for Sheik (5stock) against Luigi, Peach and Metaknight. That is just for overall training while there are no human players since online is a pretty poor option.
But if it's unpredictability you're looking for and trying to freshen those stale moves; I am pretty much master at that since D-style. The thing is when you do play the same people day in and day out they get use to your tricks and I'm sure you adapt to theirs, so you need to start looking at what does Sheik have that I don't normally use;
i.e. her chain lock more, vanish for KOs or simple movement around the map and some times don't grab the edge with the chain even though it's the obvious thing to do, mix up your get up from ledges with R/L or kick, edge dropping and then doing Fair or Nair up. Honestly, Sheik has a lot to work with when it comes to unpredictability.
Mind games such as, don't always take the easy KO. Even if they're sitting there waiting for the expected, maybe you can run away to the other edge and charge up some needles. Try to utilize Sheik's artillery of moves along with the stages, don't always fight on the same ones.
Just common sense stuff/mind games.
