If samus crouches against pikachu's Uair, she will not fall down or be sent up in the air
however, if she's in the middle of any animation (say, she's in the lag of her downsmash) and you hit her with your Uair, even if they're holding down on the control stick, they'll still either fall down or get popped up in the air
in this matchup, i use CC down tilt A LOT. samus doesn't have many options if you crouch cancel. CC dsmash works wonders against her as well. this works against samus players who tilt/jab a lot. for samus players that just like to smash a lot, it's easy to just block their smash attack, waveshield and Usmash/Dsmash/grab/whatever-you-want. most samus players that i've faught against love to ftilt and jab a bunch. crouch canceling is so excellent against them
for this matchup, i like to try to stay on the ground, approaching with running dtilts for the most part.
for her missiles, you can crouch under most of them. if she does a standing smashed missile, you can duck under it. if she missile cancels at the lowest point before she hits the ground though, then you won't be able to duck under it, but your ftilt will beat it, and you can SWD through it (YES, you can SWD through samus's smashed missile if she missile cancels it as low to the ground as possible, but you cannot SWD through it if she just does a standing smashed missile).
i've never faught a samus who likes to dtilt though. i can imagine that if i fought one, they'd probably beat me, but i don't think that most samus players use it much, if at all. for samus players that like to Utilt, if i see it coming, i like to block the utilt, then waveshield CC dsmash. if they try to jab, i'll CC it and still downsmash. if they try to spotdodge, you'll still get them. if they Dsmash though, then you're screwed lol. but if they Dsmash right after their Utilt, you'll know to not try this anymore on that particular opponent
oh and also, this is a good video for all of you to watch =)
http://www.youtube.com/watch?v=iWAEr5fsXCc