a prismatic evoker  

As a Bard, Command of Queen Veneneu Rk. I was getting resisted three out of five casts on the evoker. Once I debuffed with Harmony of Sound, mez stuck every time.

I advise that you debuff both casters (assuming you are killing the champion first) while they are running to you and casting. Then run back to your group and mez the two near the Squires spawn point.