Build and Runes
Destroy any of her tunnels you find; this will give her less map presence, as well as anyone with teleport.
Watch out for her fury bar, when she reaches Max her E does true damage.
CC’ing Rek’Sai while she’s tunneling won’t stop her. She’ll be cc’d afterwards.
Using Exhaust on Rek’ Sai, reduces both her insane mobility and attack speed passive from her ult, thus reducing the AOE hits from her Q, and making it less likely for her to retreat.