If Noxus is so strong.. How come they fell so easily to a certain ninja..

have you never heard of the Inverse Law of Ninja Strength? One ninja can easily overthrow an entire dystopian government; one thousand ninjas can all be killed on a staircase.

easiest S was 5 Yis against 5 Katarinas.

and you can 1v1 each of them without the need of ults.