Playing on PC.
My opponent cast Yue then attacked with Katara. 3 lands, 3 creatures, and trainings ground for 2, I figure the most damage possible was 8.
Opponent activated Katara for 7 and wasn’t required to tap anything, so then they activated again for 20, and had to tap 2 lands and suddenly Katara was swinging for lethal.

I can’t see anything that allows the free or discount of 18 activation.