Look up Niothedreamer Valhalla files, there's everything you need to know about Valkyries.
Personally, I never did well with Valkyries at th8, they take A LOT of skill so it'd be great if you start practising now because they can wreck th9s pretty easily (depending the base layout). Hogs are a safe bet in three starring a th8.
I think Valkyries are the trickiest troops in the game, master them and you'll be unstoppable and you'll find easy to learn other strategies involving funneling.

