If the Legion doesn't use guns most of the time, how do the survive literally any battles at all?

Simple answer is that Fallout is a setting where melee and unarmed being viable forms of combat is a baked in suspension of disbelief for the setting. Been that way since the start, when even with a heavy hitting gat you can get your ass handed to you by a guy with spiked knuckles and a leather jacket (Kane in the Hub). Any other answer is kind of dodging around that, but that's the real core of it. Just part of the pulp-fiction of the setting.

I'd also note that despite common belief, a lot of Legionaries do use rifles.

 

They use a big variety of guns:
https://falloutfanfic.fandom.com/wiki/Military_of_Caesar's_Legion#Equipment
This list is just the commonly known guns and grenades they use. It's missing the mines they also like to set up traps with (for example).

The Legion is not Anti-Gun. They just don't manufacture them, but instead they buy weapons from traders or they scavenge and loot them from enemies, old military bases, old police stations, etc. They do control a large territory with several pre-war military bases (in the real world, I don't know if they exist in the Fallout world).

 

I’d say that they are constantly getting mowed down by the NCR. It’s the fact that they just keep coming regardless can be very psychologically intimidating to the average inexperienced NCR conscript.

 

It does help that, in the end, the legion specializes in asymmetric warfare. In a war where they can just use brute force -- like with tribals -- they will and get good results, but when faced with something like the NCR they held their own fairly well despite the odds; they did lose though with the battle of Hoover Dam, and following that they waged another kind of war, one the NCR never had experience with.

In a sense, NCR vs Legion is a lot like the Korean or Vietnam wars but in a desert. Superior forces, numbers, mechanized infantry and far superior technology and training meant little when the enemy rendered such traditional or conventional victories useless or meaningless. It also doesn't help that the Legion also specializes in psychological warfare, as an army's moral is just as important as their regular old health. Whenever you can literally rip a tribe apart by enslaving the women, killing the men, and indoctrinating the children to become warriors for their Legion and only their Legion, and do so in such a way that creates fanatical zealots of the army, yah done somethin' special.

Fucked up, sure, but special, and imagine what they've got for people they want to wear down over time to a breaking point, and hit said point hard and fast. Four years of constant Legion presence against the NCR and time to work their magic; not pretty for any commander to fight against.

 

The gameplay of FNV is garbage which is why we don't see Legion use proper guerilla and ambush tactics. If a group of legionnaires lack weapons they're not gonna just run in a straight line towards people with guns. They're gonna circle around, flank them, try to catch them by surprise and strike close and fast before they know that they're there. And after they've killed them? They got guns!

 

The Legion have also only relatively recently come into a full war with the NCR on several fronts; previously they were beating the shit out of tribal gangs and disconnected frontier groups which had no such thing as standardized equipment, formations, suppy lines, logistics etc. It's why the Legion is so numerous -- they've got at least 87 conquered lesser tribes composing their ranks, and god knows how many non-tribal members they've press-ganged. They've never truly fought an actual standing pre-war style army before.

 

They do use guns, they just don't use ONLY guns.

Guns are not magical and they don't always drop people with one bullet unless its a killshot - especially with a stronger caliber. Police and military manuals tell that in a Ranged vs Melee combatant fight, if the distance between them is seven meters OR less, the Ranged Combatant is in deep danger. The shooter only has a few moments to drop the target before the target engages in CQC. Seven meters in real-life gunfights is point-blank range AKA congratulations you fucked up. There are a few "police vs crazy druggies with knife" videos where this kind of situation happens, ask any cop, they HATE when this shit happens.

Considering the Legion is pretty much an army of press-ganged, brainwashed tribals, often doing campaigns in very remote locations, it makes a lot of sense that they would be very big in melee combat. Machete doesn't jam, or run out of bullets.

Compare NCR/Legion average soldier CQC training:
NCR Trooper: Pointy end of the knife goes towards the enemy.
Legionnarie: Actual experience and training with melee and unarmed combat.

If those two run out of bullets during a campaign, NCR Trooper is good as dead. Legionnarie won't even skip a beat, just draw the machete and keep killing.

Don't forget: Legion doctrine is all about assaulting and storming enemy positions with shock attacks and asymmetrical warfare. They're not good at securing locations, they're good at taking them.

Its why Harlon noped the fuck out of the Dam and engaged the Legion in open ground using a fighting retreat covered by snipers until they lured the Legion into Boulder City. Fighting the Legion in a dam full of tight spaces is the kind of fight the Legion likes. Lee Oliver, on the other hand, wants a stand-up fight with the Legion on the Dam, and bets on the Heavy Troopers to carry the day with heavy armor and firepower.

Pity bayonets didn't make the cut, NCR Troopers doing bayonet charges once they ran out of ammo would be awesome (also it fits their "Californian Tommies" theme). I suspect the Legion would love bayonets, too.

 

In game you mostly encounter recruits and similarly under equipped fellas early on. Lorewise, Caesar is saving most of his men for the Dam itself. In game, it's pretty obvious their equipment isn't exactly balanced well. The ambush on the way to Novac where the legionaries virtually always die to the caravans passing is telling as they are actually supposed to win that ambush. Guess stats weren't properly balanced in time.

Besides that, the Legion is a force where every individual soldier is highly trained and for the most part better equipped than the NCR across the board. The fact that you see mostly recruits early on gives you a false impression of what they're actually supposed to be. The main difference between Legion and NCR is that the legion is incredibly limited in manpower and the NCR has the number advantage by far. The NCR in the Mojave is under equipped, low in morale, and suffering the affects of asymmetrical warfare unless the Courier helps them whereas the Legion is bringing veterans back just in time for the second battle. You'll note the veterans are equipped with high level weapons. Basically when it comes to quality of training plus equipment on average the Legion wins. When it comes to sheer quantity, the NCR wins no contest. It's also true that if the NCR suffers causalities they can recover pretty easily but the Legion would feel the impact of a loss more intensely due to their manpower issue.