Arash didn't fight both. Emiya handled one while Arash handled the other.
I get that D'Eon isn't a pushover, but Arash is actually pretty good, too. I forget where it was mentioned, but he did go mano-a-mano with...I think a Saber Servant using nothing but an arrow as a dagger. He's also competent enough as an Archer to target and hit Mordred's joints for disabling blows.
As for Lancelot... Yeah, Lancelot is kind of in this weird position. They're all in a weird position, because canonically, these Servants are defeated with Mash alone or the help of Mozart, Marie, or a weakened, crippled Siegfried (in other words, basically no help at all). Lancelot should absolutely be a terror on the battlefield...but he also lacks access to any of the weapons that would make him half as horrifying as he is in Zero. No Gilgamesh flinging Noble Phantasms for him to steal, no Uzis for him to dual wield, no jet fighters for him to ride or swipe the rotary cannon off of. He's also up against Bradamante, whose Noble Phantasm includes a stun effect, which leaves him open for a crucial moment if he manages to survive the initial attack.
Also, stats are meaningless. No, really, stats are absolutely meaningless. The one time they ever meant anything at all was in FSN, and even then, it was usually one specific stat at one specific moment, such as Saber's Luck when Cu used GB or Herk's ridiculous Strength stat whenever he needed to be a physically dominant presence. Stats have been absolutely meaningless as a measure of how easily one Servant might overpower another since then, or else Gawain would have shat on everyone in Camelot even more than he already did, Nightlessness or not. B+ STR is the same as Herk. If that even mattered, he would have been batting Gramps around like a ping pong ball, because B+ is still almost double A-Rank.
With Nightlessness, he should be able to solo basically everyone else, simultaneously. Numeral at its height is enough that he should be able to counter an Excaliblast with a single fucking punch, just based on pure attack power. Numeral makes Gawain into literal Saitama, and I am 100% dead serious.
Have I said yet that stats are meaningless until the narrative needs them to actually do something?
I'm not trying to come down hard on anyone out there, because this bothers the piss out of me, too, but that's just how it's been in Fate these days.