TL;DR: If I am insistent on a nation having a single weapons system spread throughout all of its troops, how much of a technological advantage is necessary to conquer the benefits of the enemy having combined arms?
I asked this Most Effective Ancient Weapons System if Combined Arms isn't an option question about a week ago, seeking an answer about which weapons system would be most effective if combined arms weren't an option. The ultimate conclusion was that combined arms will always prevail, because they can cover for each other's weaknesses and provide more flexibility, while a single weapons system will always have weaknesses.
However, for story reasons I really want one of my nations to use a single weapons system (not necessarily a single weapon, mind you, but all use the same system of weaponry). Given that it seems combined arms will always prevail given the same level of technology, I have decided this nation will need to have a more advanced degree of weapons tech to make this feasible. They are somewhat isolated, so it is completely plausible they could be ahead of their neighbors in technology, which would become apparent once they are attacked.
It is certainly true that a tech advantage with one weapon can beat combined arms. For example, we could have a caveman tribe with a combined arms military, made up of Og, who hits enemy with stick, Bog, who throws stone at enemy, and Pog. Pog use teeth. The caveman army has fifty of each. On the other side, we have fifty US Marines, all armed with modern rifles. While one side is significantly larger AND a combined arms military, there is no reasonable argument for them having a tactical advantage in any foreseeable battle situation.
However, this wide of a technological gap doesn't make for an interesting story. Therefore, what is the smallest tech gap that can make up for having only one weapons system, preferably not going any higher than medieval technology (as this is a fantasy story)?
Current thoughts include an army of bowmen while the enemy nation has no armor, or hoplite style large shields with armor while the enemy is clad without armor and largely dependent on projectile weapons (many historians say this is a primary reason the Greeks won the Persian wars, as the Phalanx was especially well suited to the terrain, as well as a counter to Persian tactics). (And yes, I know the Greeks had more than just Hoplites here. The point remains that the famed Persian bowfire was reduced in effectiveness by hoplite armor, and the Persians were not prepared to take them on in hand to hand fighting.)
Again, I recognize combined arms are better. Seriously. I get it. But I want this army to have one weapons system, so I'm going to give them a tech advantage. If you still don't think they can beat a combined arms enemy, then you can stand with Og Bog and Pog, while I go with the Marines, and we will see what happens.