Haven't gotten them all yet, still backtracking to earlier levels, but one tricky stamp for me was the Hands-On Hall one. I didn't recognize that the rolling projectile the Spikes were throwing weren't breaking on the "wall", so I had no idea that I could walk outside of the castle at that part.
Formerly known as Hero Fly Chao. 3DS FC - 0087 2307 1826 "Dex I love you but sometimes you make me wanna vomit" - vidgameking84
Hands-On Hall's and Clear Pipe Cruise's stamps I only found out how to get after finding Miiverse posts about them. The Green Star in Faster Fire Bros. Fort where you have to ground pound the shiny portion of the ground took me forever to find. It felt like they just couldn't think of anywhere good to put one after already having two in the stage and an earlier version of the level with its three so they just decided to stick it underground.
'Official' Plusle of the SSBU board. Also, creator of that thing down in the line under this. http://www.gamefaqs.com/boards/997828-paper-mario/57781633