Given your existing options (specifically UDK for attack down) I don't know if we need another attack down option for clan boss, unless you go the Coffin Smasher route (which wouldn't be bad at all, I think.)

Lightsworn at 40/50 (depending where you are in FK) would be perfectly fine to bring down the shield and keep turn meter down and in theory with Armiger/CH should be enough to keep FK's turn meter down. Are you dying on the boss fight, or having trouble with waves?

Also, where are you in FK, CB, and Spider? 

dragon 20, spider funny enough I just got 20 because I was able to finally fuse my Walking Tomb during this past fusion event. Omg WTD trivialized Spider for me. I was stuck on Spider 15 and then after just putting WTD and UDK in i was amazed how easy it became. 

FK I technically beat at 20, but it took 10 minutes manual. The largest problem I had is that stage is force affinity and my "usual" team comp is like Gorgorab, Uugo, Kael, Royal Guard, Coldheart. I tried adding Apothecary in but he's 50 so he basically got 1 shot during waves 24 7. 

FK is kinda challenging for me with just a bunch of single hit champs outside of CH. 

For Clan Boss Im 3 keying Brutal for the 21M total. Id like to get into NM / UNM but I realize my account is still fairly young. 

Lightsworn was one of my first 60s (I got him from a promo code). He's a strong single-target hitter, and in your case benefits UDK with def up. Also great in faction wars.

Definitely worth building at 50, you can decide about 60 from there.