Drop his shield, apply debuffs, and make sure you have high accuracy

How can I drop his shield?:)

You need to hit him multiple times. There is a counter near his hp gauge that goes down everytime you hit him. Once it drop to zero, you can now apply debuffs to him. So, basically, you need multi hitter champions to bring down his shield quickly

hit him more often than his Shield has charges.

it's a stupid Thing. As FK is Pretty hard without the Right Heroe Combo (coldheart with her 4-hit A1 for instance) it's not a nice Weekly Mission. 

If you want to get it done easy and fast take 5 People who can apply heal-debuffs on their a1/a2 and 2 of them hit at least twice (or three times). Drop the Shield, apply heal debuff 1-3 times before he upps his Shield again, rinse and repeat.

I went down to FK 5 I think, as that is still Pretty easy to beat even with a suboptimal Group and applied 8-10 heal-debuffs per run of ~1.5 Minutes that way. Stupid, especially as I didn't Need any of the gear he Drops on that lvl and even the selling-silver is not a lot, but at least it works and can be done in less than 30 minutes this way.

Thank you both for the help and good advices! :) I´ll try this :)

Team CA with multihit A1s or someone with reflect damage on all expedite getting the shield down. Then applying the heal debuff just needs ACC, if you see resist your ACC is too low.  Post the team you are using with level if you still are having trouble.