Upgrading a number of guardians is boring and takes a lot of time. It removes focus from the fun parts of the game. The main problem is scrolling, when you have many slots. To upgrade one guardian, you need to scroll two times and click a number of times. Then the same for the next one.

Proposal: make two windows side by side, one window with the list for the guardians to be upgraded and one for the guardians to be sacrified. Once a guardian is upgraded/sacrified, it disappears from the list, and the next in line replaces it on the list. In this way, scrolling is needed only for the first upgrade.

Niels