05/06/2022 - I've heard that the now tower has only 2 merchants, I do not know if thats true because I did not play the game after 100%. If anyone knows how to complete the achievement with new tower you can comment and I will past your comment here.
There are 3 tower's vendors, each of them is located before boss fight, you need to come and interact with them by upgrading any item.
I will describe you how the tower's levels are organized:
1-8 are combat levels you need to fight some enemies,
9 is merchant/vendor level,
10 is boss fight,
11-18 are combat levels you need to fight some enemies (again),
19 is merchant/vendor level,
20 is boss fight,
21-28 are combat levels you need to fight some enemies (for the last time),
29 is merchant/vendor level,
30 is the last boss fight.
Achievement should pop after you upgrade at 3rd vendor's "shop"/level
Achievement also use tracking so it will count 33% for each vendor you interact with
Good luck!