-Always try to keep your opponent at Zair range.
-At larger distances (and given they aren't Sonic, Captain Falcon, or Sheik) go for a short hop homing missile to create space.
-ALWAYS try to punish your opponents unsafe aerials on your shield with an OoSO (Out of Shield Option), because Samus has plenty and they are VERY good. She has: Usmash, Nair, Jab, Dtilt, Ftilt (angled if needed), and especially Up B (arguably her best one). At high enough percents, the Up B will kill lighter opponents especially with rage. If you are playing with Tap Jump ON you only need to
quickly press Up and B. If your Tap Jump is OFF, you need to JC it (Jump Cancel). You do this by pushing your jump button and
quickly sliding to your B button (or whatever button you have set to special).
-Try to utilize SMR (Super Missile Rush) against dash happy opponents. If one of your SM catch your opponent, you sometimes get a follow-up depending on your range and percentage. (Ex: SM>Zair, SM>CS (Charged Shot), SM>Nair).
-SMR input: simply do a smash side B (SM input), dash forward once by flicking the stick, then do another SM input, rinse & repeat. It should be a somewhat rhythmic sequence of inputs, one after the other. Here is a visual:
/+>/(same direction as initial missile)>/+
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SMR (Super Missile Rush)
These are some of the tools I use. Hope they helped.