So in this battle, I used leaf storm. The -2 happened, and when I used leaf storm again, it still did 70% of its health. There was no crits, and I really doubt I got very low damage rolls than a high one. But what really happened? Thanks
Roost caused Mantine to lose the flying type, making it mono-water and weak to grass. The -2 drop effectively cancels out the super effective bonus, making it the same damage.
Hope this helps!
Because the mantine used roost.
Roost eliminated its flying typing, and eliminating the flying type made leaf storm Super Effective. The flying type previously made grass neutral.