Looking at this, I assume the plan is to lure a Stille to a pond filled with Kanne's magic, while under normal circumstances a Stille would be deterred from doing so, by freezing the lake they remove the largest source of water available in the testing area. Lawine would then go back to where Kanne is to prepare to capture the Stille again using the same method as before.
Frieren may be by herself, to frankly unleash the amount of magic she has instead of hiding it as she usually does, to deter any of the Stille from going near her sort of drawing them towards the area Kanne and Lawine are where the Stille would have no choice but to drink water from the magic filled pond. Unlike last time if Lawine manages to freeze it cleanly, then Frieren can come in and use her bird restraining magic.