@Diatenshi
Here are some useful things...
-If pika tries to spam, even if it's just 2-3 bolts(i'm referring to bolts as pikas b-move since i forget what it's called) once in awhile, just do a short hop over it and missile cancel or zair if you're close enough.
-Short hop fast fall zair is also very handy in this matchup since pika's usually like to use their speed and size to punish samus' short hop, and shffz hits them before they can do anything. Just don't use it too much because they will start to see it coming and run up, shield, and grab or any other oos option.
-like Throwback said, f-tilt is good because it outranges and it's fast. Don't just walk up an f-tilt though, use it as a gtfo option and wait til pika is on 1 certain side of you instead of just throwing it out while he's above you, and hoping it will hit. If pika does shield pressure with jumps, up-b OoS works well.
-Since pika is light, aside from the blast, try making d-tilt and d-smash your primary killing options(unless they go to the edge a lot, which is a bad idea against samus) So try to camp them until they have high percent, and go for the kill with d-tilt/smash. Don't rush it or use it too early, since you should try to keep those moves fresh, and also rushing for the kill never works with Samus.
-Whenever you have the chance, start charging your blast, because when that move is charged pika can't rely on his size to avoid it, obviously.
- as for hitting pika, which seems to be your problem as you mentioned, Just try to camp him out with missiles zairs etc. and read things like quick attacks and aerial approaches, and punish after you read. You have to avoid getting yourself into situations where pika's size become an issue like when he's all over your shield and you don't know where/when to hit. In case this situation DOES occur, up-b OoS works well as a backup.
I had pretty bad grammar in this because i'm kind of in a rush but it's no big deal
lol
I hope this helps.