Just a simple little illustrated guide on the proper technique to unlock the 5 secret achievements.
In the very beginning of the game, play up until you get to the cave with the hole in the bottom, but do not jump in. Examine the hole then choose "no" when it asks if you want to jump down, then backtrack to the bridge with the bodies on it. Position yourself as close to the middle-most corpse as possible (as in the screenshot here) and hold the Absorb Souls button - it can take anywhere from 3-30 seconds or so for your geese to pass by. If you go too far and the camera angle changes, simply exit the area through the house the demons were in and re-enter the bridge area.
As with Fall's Flight, play until you get to the cave, examine the hole without jumping down, then return to the screen with the three humans. Turn around and go back to the screen and position that you see in the screenshot, and hold the Absorb Souls button. You will shortly see a big beetle climb onto the right tree's trunk and shortly afterwards will get your achievement. Alternatively, if you're already doing Fall's Flight, just go do that one in its entirety, then truck on back to the cave and do Undying Summer on the way to the rest of the game. I'm not the boss of you.
Later in the game, in the West Area you will encounter a small bamboo forest, with several bamboo shoots that are lower-resolution than the background. You are forced to go through this area as Kaede so as long as you remember to cut down one of those bamboo shoots (either with the dagger or a kunai) you can't really miss this one. If you did miss it as Kaede, you can go back as Samanosuke.
All Lives Big and Small
In the East Area, you will find yourself in an area with many bridges over water. Continue progressing normally until you reach the area as seen in the screenshot, with several small koi fish swimming around. Stand in a portion of water you've seen the fish go through and hold the Absorb Souls button, and eventually when a fish gets close enough you'll absorb a small yellow soul - and the achievement.
This is the only genuinely hard one to get, because it requires a perfect parry on the second-to-last boss. During the fight, he will eventually draw another sword and begin a new attack pattern. You must parry one of his attacks - so perfectly time a block, then immediately do a normal attack afterwards. It can take a few tries but it's doable. Doing just one of these parry attacks will unlock the final Secret Achievement, and you'll know you've got it if the screen flashes when you strike him (or obviously when the achievement pops up). Good work!