This would need Trial, right? What would you use over Mordred if you had Trial ex?
I thought about it a bit. Without testing with Mars it's hard to judge whether Vicissitudes is needed (Mars can miss, Amaterasu's blind can miss) and there's the possibility of Ambush being reliable with resist down + VoF.
By removing Ambush def down drops to 45%. Hercules/Shingen can get back to 50% with the Pride bow and they have more offense than Mordred. Hercules doesn't need an ex skill so that allows Trial, Full Break is also nice to have. But that leaves the issue of the Pride bow having worse accuracy than Ambush, and Propaganda potentially being a better defensive skill than Trial. Mordred still seems more reliable on paper.