But he does it at Gil's level: When he betrays Caster on UBW he basically skewers Caster in the very same fashion that Gilgamesh does in the Fate route, while still injured from his Lancer duel. No, I must say, he does it more efficiently than Gil, and then makes a prison of swords to keep Tohsaka at bay while he attempts to kill Shirou, even to the point of casting UBW. Then, we have him firing many swords making a path for Tohsaka again to escape from the Giant Blob Grail, all of this while out of mana and with no Master, with Gil's caused wounds still present... he even traces a Rho Aius for Shirou to cast UBW unhindered, doesn't him? I personally think of Archer as a battle hardened and experienced Gilgamesh, that doesn't constantly subestimate his opponents, can hold his own against other servants on CQC, has a better but harder to cast version of GoB but lacks Ea and Enkidu.
If Gil vs Berserker was a win for a standing still, arrogant Gil that instead of retreating on the very last moment summoned Enkidu, then Archer actively looking for a better position for attacking Berserker while not caring about his arrogance or the image he gives for the GAR fanboys then would have a much better chance while mass tracing swords: If Gil was saved by Enkidu, in the very same fashion he could have retreated and used more swords instead of standing there arrogantly and chaining Heracles: thus this path could be followed by Archer as well. Also, we can safely say that while Gil has one example of every original model, Archer has the originals and the derivatives, and every weapon he saw in his time as a Counter Guardian (and, knowing that he's sick tired of being one, there must be A LOT of them).
A last word: while it DOES say that "the duplicated weapons has its rank reduced by one" on UBW description, UBW's rank its still the same as Gate of Babilon: E-A++ I assume the reason for the variable rank is because the weapons inside both NPs varies on the same fashion as well: if it's so, then both Archer and Gil have weapons of the same level, and the "reduced rank by one" is pretty much figurative. We all saw how in the Shirou vs Gilgamesh battle both Gil's originals and Shirou's copies clashed and destroyed each other: that seems pretty much like same level to me. "There's no rule that the copies must be worse than the originals", Shirou said...
This all is spoken while assuming that Archer has no time to cast UBW, and instead uses mass tracing (that's basically making the images already present in his mind exteriorize in the outside world, instead of expanding his mind and replacing the outside world with it). In UBW, I'm sorry to say this but IMHO no 5th Holy Grail War servant would be much of a match, provided that he has enough mana to maintain the thing active... He does control and can levitate and launch at considerable speed all the INFINITE swords inside, right?