1. Upon entering the cave, behind you will be a Phoenix Down.
2. Walk straight ahead for a Potion next to a fire pit.
3. After you “proceed” through the tunnel, there’s a Potion next to the second fire pit.
4. When you hit the waterfall, go left to where the water is coming from for a Hi-Potion.
5. After fighting the giant snake, and then Gilgamesh, turn around, and head up the rocks to your left for a Potion.
6. If facing away from the waterfall, go to the top left corner for a Potion
7. Look to your right from the previous pick-up and across the water there will be another Potion.
8. Yet another Potion can be found on the left, just before you “climb” up the edge.
9. After you “climb” the edge, there is a Potion to your right.
10. As you head up the stairs, there will be a Hi-Potion hidden in the rocks on the right.
11. Soon you’ll come to a camp where you can rest, and a Potion will be in close proximity.
12. After a battle with some enemies, in a dead-end to the right, you’ll find a Potion.
13. Before heading up the stairs, go left into a dead-end for a Hi-Potion
14. After you “climb” the ledge, head up the stairs, and then to the left to find a Phoenix Down.
15. After crossing the rock-like bridge, there will be a well hidden Potion next to a barrel on the right.
16. After crossing another rock-like bridge, you will fight 2 Reapers, there is a Potion on the other side of the rock formation on your left, this one is easy to miss.
17. After dropping down the stairs and engaging more Reapers in battle, there is a Potion to your right.
18. If facing away from the stairs that you dropped down from, there will be a Potion in the top-left on top of some rocks.
19. After you “climb” the ledge, on the right very shortly you’ll come across a Hi-Potion.
20. After clearing the trial, you’ll come to a camp where you can rest, there will be a Phoenix Down in close proximity.
21. Before heading up the walkway, there will be a Potion in the rocks to the right of the walkway.
22. After traversing the walkway for some time, you’ll come to a section that separates 2 different walkways, a Potion will be to the left, it’s just after the part of the walkway that you can’t ascend because it’s become dilapidated.
23. Before you “climb” the ledge, there will be a Hi-Potion to your left.
24. After you “climb” 2 ledges, go up the stairs, and there will be a Potion on the right.
25. After fighting some enemies, 2 more enemies will appear, there is a Potion on the ground where they appear, it’s just to the left of the stairs that take you to the next area.
26. After you “proceed” go up the stairs and when you’re at the top, there will be a Potion to your left, next to the dead warrior.
27. After Beating Enkidu, “climb” the ledge, and there will be a Potion to the far left.
28. After fighting some enemies, you’ll go down some stairs, and should be able to see a Potion on a pedestal, it’s at the part where the staircase turns.
29. After descending all of the stairs and fighting some enemies, if you look toward the stairs that you came from, you should be able to see a Hi-Potion on a rock--adjacent to the staircase.
30. As you exit into the open (where The Soul starts talking) there will be a Potion to your left.
31. Travel along the walkway, and before it starts to descend, there will be a Phoenix Down next to some barrels.
32. Descend the walkway, and after making 2 turns, there will be a Hi-Potion in the rocks on the right.
33. When you make it to the bottom of the walkway, before engaging the enemies, turn around to see a Potion next to some barrels.
34. After killing Bandersnatch, before heading up the stairs, there will be a Potion in a dead-end on the right.
35. After fighting some enemies, before going up the walkway, turn around to see a Potion next to a pillar.
36. Instead of going up the walkway, go to the right of it to find a Potion on your left.
37. Where the camp is, climb the rocks to your right for a Hi-Potion.
38. After going up the stairs after the camp, look to your left for a Potion.
39. After beating the Demon Wall, before entering the next trial, there will be a Hi-Potion near a column and some dead warriors.
40. In the same area as the previous pick up, there’s a Potion to the right, before you enter the trial.
41. In the trial area, as you go down the stairs, keep your eyes forward, soon you will come across a Phoenix Down, it’s on a staircase that doesn’t lead anywhere.
42. From the previous collectible, turn around to see a Potion on top of the rock.
43. From the previous pick-up, go down the stairs and in a dead end to the left is a Hi-Potion.
44. After the camp, head down the walkway, at the bottom, there will be a Potion to your left.
45. Go down the next walkway, and when it ends there will be a Potion in the rocks on your left, be careful to not get too close to the objective before picking it up.
46. After the cutscene, turn around to find a Phoenix Down on top of a rock.
47. As you go down the stairs, keep going straight ahead for a Potion in the rocks.
48. Before heading down the stone-like path that leads to the final battle, look to the right where all of the columns are for a Hi-Potion.
The achievement should unlock shortly after picking up the last collectible. Seeing as you start with some items already, if you pick up all items and never used any of them, you should have: 51 Potions, 21 Hi-Potions, and 11 Phoenix Downs (if you're on easy).