3. Uh... Who's the 3? The prep makes a difference so he probably takes this one.
I really don't think GS has the stats to keep up with Mugen for very long, he usually gets his ass kicked by stuff that he should be defeating like the Goblin Lord and Goblin Champion. His willpower is pretty amazing and it allows him to pull through some pretty heavy wounds, but that won't help much when Mugen chops his head off.
However, GS is incredibly crafty, and he could potentially kill the entire Champloo team with prep if he ambushes them.
GS with prep and knowledge in a 1v1 could take a 5/10 at best IMO.
Mugen would win against most silver ranks tbh, I'd argue he could stand toe-to-toe with a Goblin Champion. Yes, Mugen would've died against Oniwaka if Fuu hadn't said anything, but consider he was wayyyy faster then the Goblin Champion, Mugen was badly poisoned by something that was going to kill him in a matter of hours, as well as being in "agony" the entire time. Despite all this he still put up a good fight and was able to continue performing superhuman feats left and right.
I think you are down playing the fact Goblin Slayer is fully encased in armor, he has chain mail under the leather too. And that includes his neck as shown in his first appearance where a Goblin stabs him in the neck with a poisoned dagger and can’t get through to him.
Spearman and especially Heavy Warrior have magic and magic items respectively but GS tanked hits that shattered stones. Witch has literally no noteworthy physical feats and Female Knight has only ever been Heavy Warriors equal after he exhausted himself and it was played as a joke. I agree Mugen is physically > but GS has full body armor, poison, healing potions, stamina potions daggers, a sword, bow and arrow, a sling and a shield as standard equipment.
Also you do know Priestess’s barrier is separately permeable? (Ie. GS can attack through it but not his enemies)