Siegfried is here in the latest 1AP, and he's as OP as the lore makes him out to be. Specifically, he's got a 200% defense boost. Most attacks, even with class advantage, will do 0 damage, even with event damage boost CE. The buff can't be cleared, so you have two options -
1. Defense-bypassing NPs
Not many of these. EMIYA is apparently the only Archer with one. Other hard hitters are Jack, Okita and Kintoki. However, they still don't hit too hard - they only bypass 100% defense.
Edit: List of defense-bypass NPs from reddit:
EMIYA, Okita, Saber Shiki, Assassin Shiki, Hector, Mephisto, Phantom, Nero, Kintoki, Artoria Lalter, Elizabeth Lancer & Caster, LuBu, Astolfo, Spartacus
2. Defense debuffs
Skills, NPs, what have you, just spam them on damage turns.
I used a mix of these. My NP3 Emiya was MVP, spamming UBW like there was no tomorrow. I grabbed a Jack support and popped Shuten in for debuffs, and Mashu to keep Emiya alive. Her defense buff and NP make Siegfried hit like a wet noodle, especially combined with EMIYA's attack debuff. You'll need it, since he fires off his NP on the first turn. Chip off his 50K HP, and job's done.
I found this by far the easiest challenge so far. Essentially a two-servant job.
I've also heard that Buster chains can also overpower his defense due to some quirk in calculation.
P.S. Shameless plug - my EMIYA is in my support archer slot with an event damage CE if you guys need him.