If you've ever attempted a Raid in any RPG, then you know that there can be no room for error, for even the simplest errors can lead to a wipe and the loss of hours of progress. As such, every prospective Raid member must be at their best, and that means that you may have to spend days, weeks, even months to prepare for a Raid, grinding away at an activity until you get that one perfect weapon or armor piece that will help you survive just a bit longer or do a little bit more damage. That is the harsh reality of how Raids work, and that maxim remains true in Destiny 2's Leviathan Raid.
Fortunately, Destiny 2 shows a small degree of leniency in that while certain specific weapons may be considered a must-have for the Raid, there are a variety of viable, relatively easy to obtain weapons that you can grind for.
Must-Have Exotic Armor
Lunafaction Boots (Warlock)
Aesthetics aside, every Warlock should pursue the Lunafaction Boots simply because of the enormous utility that they provide to the team. While wearing the Boots, Warlock Rifts will automatically reload the weapons of every teammate who is standing in the Rift, which means that you and your team can unleash an endless barrage of bullets so long as the Rift exists. Unfortunately, you will have to (rather literally) jump inside and out of the Rift for the Boots to actually work, but there are a few saving graces; merely activating the Rift will instantly reload all of your weapons, so you and your team can empty all of your guns on a target, drop a Rift, and keep shooting.
If your Titan's half-size barrier is on cooldown and your team needs a quick reload during a DPS phase, drop a Rift. Multiple Warlocks with multiple sets of these boots? Stagger your Rifts for the different plates so that you can all but guarantee an extra initial volley on each plate. Part of the Shadow Realm team? Maximize your skull killing potential with a Rift. Of course, damage buffing Rifts should be deployed as early as possible, but with the Lunafactions, you can heal and provide up to three free reloads per person on demand. Coordination is key to maximize the efficiency of these Rifts, but there are literally no other Exotics in Destiny 2 that can be more helpful in the Throne room encounter (which, admittedly, isn't saying much considering how niche some of the other Exotics are), as it forces you to group up on a plate if you want to achieve maximum DPS.
Celestial Nighthawk (Hunter)
This Exotic Hunter helmet is a paragon of what Exotics should feel like in Destiny 2. While wearing it, any Gunslinger Hunters who use their Golden Gun will find that they will only have one shot, but that one shot will often obliterate anything in its path. Land a headshot and even the toughest and largest enemies will fall before you. For practicality's sake though, it may be for the best if no more than one Hunter on your Raid team wears this, as it offers very little utility for the team; having a combination of Gunslingers and Nightstalkers is a bit more ideal than a mass of Gunslingers, especially if they don't all have access to a Nighthawk.
Extremely Helpful Exotic Weapons
Coldheart (Trace Rifle)
This odd freeze-ray may seem rather unwieldy at first, but it shines during the boss fight with Calus (or for that matter, any fight where the target is large and stationary), especially if you are responsible for destroying skulls in the Shadow Realm. There's not much to say about the Coldheart because you point it at something and hold down the trigger until the gun's ramping damage cuts through the target or you have to reload. Unfortunately, this weapon is exclusive to those who pre-ordered Destiny 2 until December 5, 2017.
Merciless (Fusion Rifle)
If you're looking for a versatile weapon that is useful for every part of the Leviathan Raid, look no further than the Merciless. It may not be as flashy as some of the other Exotics in the game, but what it does have is a perk that can help increase the amount of DPS you provide. As long as you hit a target and it doesn't die, the charge time of Merciless will decrease, eventually turning Merciless into an instantly-firing Fusion Rifle. This perk resets on a kill, but that is rather irrelevant when most of the priority targets in the Raid (bosses, Bathers, Standard Bearers, etc.) require multiple hits anyways.
The Best of the Rest (Exotic Weapons)
The Prospector (Grenade Launcher)
A new weapon class to Destiny 2, grenade launchers are great for throwing out a lot of damage in a short amount of time as long as you don't really care about where that damage is going. Nowhere else is this more apparent than in the existence of The Prospector, which is basically just a fully automatic grenade launcher. Not especially great against bosses, but no noteworthy weaknesses either.
D.A.R.C.I. (Sniper Rifle)
If you must use a sniper rifle, at least use one that offers extra headshot damage. That is more or less the only reason why D.A.R.C.I. exists, though there are better, more versatile options for the majority of the Raid.
The Wardcliff Coil (Rocket Launcher)
Generally speaking, the Coil is not that great. It's a rocket launcher that fires multiple projectiles, but most of those projectiles miss anyways unless you're dangerously close to your target. However, if your target happens to be a large, stationary boss that doesn't use an insta-kill melee attack all the time (i.e. Calus), and you have a Titan with a half-sized barrier or a Warlock with Lunafaction Boots, then you can use the Coil to dump an enormous amount of raw damage onto someone. Be careful not to shoot a teammate in the back with this while on the plates in the Throne room though.
Sunshot (Hand Cannon)
Readily available from the main questline, the Sunshot is a decent choice for clearing standard enemies away from a point thanks to its combination of explosive bullets and its ability to turn its victims into a miniature grenade. The magazine size is far from ideal, but if you're looking for a backup weapon in the Castellum, there are far worse choices than the Sunshot.
Must-Have Legendary Weapons
Nameless Midnight (Scout Rifle)
There is no reason for any member of your Raid team to not have this Scout Rifle in their inventory, as it is easily obtained by doing two Strikes for Zavala. The Nameless Midnight offers explosive bullets, high ammo efficiency, respectable damage (enough to put down a Legionnaire within four or five body shots), and a decent rate of fire, all of which are necessary traits for the Raid, so much so that the Midnight should be considered a standard Raid weapon. The only area where the Midnight is at a clear disadvantage is if you are responsible for shooting skulls in the Calus fight, and even then, you should have an automatic backup weapon if that is the role that is assigned to you.
The Best of the Rest (General Loadout Tips)
If, for some reason, Destiny 2's RNG-infested loot system really hates you and you just can't find any of the aforementioned weapons, keep in mind that there are plenty of weapons that can be used as replacements. As a rule of thumb though, every member of the Raid team should have either a Scout Rifle or Hand Cannon on them as a primary weapon. Pulse Rifles are also acceptable if they are of the slightly faster-firing variety like Nightshade, but you should be mainly looking for precision weapons that offer a high critical damage modifier to efficiently dispatch standard enemies and to give you some extra damage against bosses. It goes without saying that weapons like Auto Rifles and Submachine Guns should generally be avoided unless being used in a secondary manner due to their inherent inaccuracy and low damage per shot.
Similarly, it's not that great an idea to bring Swords to a gunfight, unless you're using them as a point-blank defensive tool. Shotguns and Fusion Rifles are simply far more versatile, and they can be used without putting you in direct danger, which is usually a good idea in Raids since losing a teammate can cause a chain reaction of death among the Raid team. Sniper Rifles, while a decent choice in theory, are much too unwieldy at close range and when facing down multiple enemies, both of which are problems that are exacerbated by the aggressive fighting style of the Cabal. Sidearms aren't a terrible choice, but they lack range, and you want to be consistently effective at every range, especially since ammo in Destiny 2 is much more of an inconsistent drop.
However, that's not to say that if you find a weapon that you are comfortable with in a given situation, you shouldn't use it. The Pleasure Gardens, for example, encourage close range DPS, so burst-fire sidearms and submachine guns can be used to help hose down the War Beasts. Above all, keeping a diverse array of weaponry on hand should be your top priority when grinding, as you never know if and when you will be called upon by your Raid team to run the Gauntlet, or carry the Standards, or enter Calus' Shadow Realm to call out icons.