Before someone else butchers their plagiarised form of my build again by trying to apply tabletop rules to BG3, this is how you'd build the highest damage archer under BG3 rules. Though it's a quadclass at most here rather than the pentaclass it is in tabletop.
Primary: Gloom Stalker 5 (Take these first)
Secondary Classes: Assassin 3 then Spores Druid 2, in that order
Finisher: Either Gloom Stalker 7 or Fighter 2 or Assassin 5
Race: Any
You will only ever get one ASI at level 4 Gloom Stalker, which must go into Sharpshooter because elevation matters a lot. It goes without saying the Fighting Style should be Archery.
Required stats: 8 Str, 16 Dexterity, 16 Wisdom
Optional stats: You can get more Constitution if you want, though if you're positioned correctly you're unlikely to be hit anyway so this is more for keeping Hunter's Mark up against stray shots. I'd recommend dropping points into Int or Cha for the skill checks this game loves so much.
How it works:
With Spores Druid 2, you can raise a 8tHP shield that gives you +1d6 Necrotic per hit. This normally only buffs melee attacks in tabletop and gives rise to the strength-based Druidin build there, but for some reason it works with ranged attacks in BG3 as well.
Get yourself blessed by a party member to make Sharpshooter land more reliably.
Gloom Stalker 5 gives you two shots normally, and a third on the first turn of combat that gives you +1d8 damage on top of the usual damage..
When initiating combat before being seen, you'll fire one shot with advantage from stealth that autocrits due to Assassin adding Sneak Attack, then get back that shot due to Assassin, before the first turn proper starts. That's 2d8 + 6d6 + 13 (43) off the bat.
Then stick bonus action Hunter's Mark and shoot the three times for the first 'surprise' turn, dealing 2d8 + 4d6 + 13 (36) twice and 4d8 + 4d6 + 13 (45) for the third. At this point before anyone else except your Cleric and your MC have taken any actions, you've already deleted 160 HP on average, possibly less if you missed.
On later turns you will hit 1d8 + 2d6 + 13 (24.5) twice, with a 2d6 (7) floating sneak you can stick on either attack. If you really need to you can use the bonus action to hide to get advantage for sneaking, but often your party members when positioned correctly can qualify for this for you. Anyway, 56 damage ish per turn after that, using bonus actions to either shift Hunter's Mark or to hide to qualify for Sneak.
Fighter 2 gets you Action Surge, which doubles your first turn damage (+117) once per short rest, and can also get you +1 AC from the fighting style and Second Wind, which combined with the spores from Spores Druid means you'll basically never really need the healing from the short rest button even though the rest of your party will. In most cases, the 277 damage here deletes any boss, though it's more likely you'll delete 4 targets instead of 1 boss in most encounters as most boss class fights are enforced dialogues that don't really allow Surprised to apply to avoid this kind of BS.
Gloom Stalker 7 gets you proficiency to both Intelligence and Wisdom saving throws, on top of the Strength and Dexterity you start with, as well as a third level 2 spell slot, which is good for Longstrider, Darkvision, Spike Growth or Protection from Poison.
Assassin 5 gives you one additional 1d6 die on Sneak Attack, allows you to get Resilient Con if you want (in which case leave Con as an odd number), 18 Dex if you want
though a particular set of gloves will raise it to 18 anyway
, and halve incoming damage with Uncanny Dodge once per turn, allowing you to have a higher chance of not having Symbiotic Entity broken.
If you do this correctly and position yourself well, it's possible to delete most encounters from 18m away quite easily on the highest difficulty before enemies have any option of doing anything except Dash (possibly through a Spike Growth field if you prefer that to Hunter's Mark).
The problem is the missed RP opportunities from dialogue you can get from walking into the encounter's range, but if you're not planning to parley anyway you can always shoot first and shoot more later.
You are also warned that using ridiculously broken builds like this may detract from gameplay, and you may want to use tamer partly optimized builds like heavy armour monks or pure spores druids instead so there's some level of challenge.
