So im just wondering how to counter him if u go jungle? Because his wraith skill does 504 dmg or so with only bladestalker on, thats like half of your hp early game, lol. His mobi skill and knock ups are so op, and wait til he gets to late game he is beast! dmg+tanky so op lol.
So yeh I was wondering how to counter him if u are going jungle against him, like how to slow him down, and if he got fed somehow is it going to be the end of the game?
Did you mean "Her"? 
is a female. Anyway, here's my opinions about her:
I've encountered a Rek'Sai as Sejuani. I stunned her with my ult when she attacked to my carry, and let my team kill her. If she E's, go Q her and she should meet her death with your W (Hi percentage max HP damage on on-hit+an AoE scales with Sej's HP). Also stop her with building armor, because Rek'Sai's Q deals physical damage and it's an on-hit effect so it's affected from AS slows.
I've stopped a full-built Rek'Sai with
and a 
when I was 
.
But, I didn't stop Rek'Sai as 
because Shyvana has too little CC for stopping her.
TL;DR Rek'Sai is Shyvana v2. She is like an assasin. Go CC her, and let your team kill her. She doesn't have too much tankiness or CC.
