is there a reason to 100% the libary and what kind of stats are needed to fight the winter witch and knight
The witch isn't all that difficult, just stack Ice Resistance and MDEF while refraining from using magical attacks due to reflection.
The number of knights around in that stage however, and the fact they can prevent escape at <20,000 AGI makes them a nuisance. They use exclusively one percent-based attack (dealt with using Whittled Ring and Sho's Pledge) and have a defense buff which reduces percent-based damage you deal and also fully heals them when dispelled.
Thinking back to what others said earlier, I think they're intended for DPS-checks or stunlock capability on a certain final boss.
The strat which also applies to their respective boss:
Kishingatana/Deep Sea Anchor/God's Angels x7 Stack, Golden Mead, full Resin set or Element stack with Royal Tea, Omnibless x2, Omnicurse x2, King's Order, anything else you can think of.