Terrain Seiryu

[Renato]

I have a question, what exactly does this Terrain Seiryu do? Does all skills give normal regardless of type?

[Spar]

Pretty much. It negates any type interaction, at least to the extent that I can tell from having used it (somewhat accidentally) on a couple occasions. I think STAB still works as normal, but I haven't checked thouroughly. I don't know if status moves work on types that should be immune (Poisoning a Steel type, for example).

[Renato]

I used this terrain against my brother and he attacked acid tears in my extra rika and it did not work. I also tested with akyuu Frail Health and unfortunately did not change anything :c
I thought she would be invincible ...

[Spar]

There might be some sort of background check for Frail Health to prevent complete invincibility. I've only ever fought under conditions of any Terrain or Weather when an opponent summons it, and I usually just ignore the effect as it very rarely causes any real problem.