If it's just 50 Daleks in a stadium, Illyria will rip them to shreds. 

If the Daleks have access to their technology, it's another story entirely - Illyria would still be a tough nut to crack, but 50 Daleks with their weapons are a threat on a universal scale.

Illyria is at her fullest strength and in Fred's body
She has the power to slowdown time and has super strength, She is almost invincible.

There are 50 Daleks all with lasers that can kill just about any life form, they can fly and their exterior is almost impossible to break. (If we are talking new series.)

Illyria "Angel" vs 50 Daleks "Dr. Who"