It’s possible (though incredibly difficult) to complete Baldur’s Gate 3 with a single character. Some classes for a Solo Challenge run though, are much better than others at getting the job done.
Baldur’s Gate 3 gives you many options for a Solo Challenge run, as there are tons of items and environmental effects that can be used to give you an advantage over groups of enemies. Just remember that Baldur’s Gate 3 uses the Dungeons & Dragons 5E action economy, so being outnumbered is a massive disadvantage for a player character.
A Multiclass Rogue/Fighter & Rogue/Barbarian
There are a few variants of this concept, but a Rogue + Warrior-type is the overall gist of the idea. Rogues have access to lots of Skills, which you’ll need with only a single character, and their Cunning Action is incredibly useful for evading Attacks of Opportunity, crossing a lot of distance quickly, and hiding in the middle of combat. They can also deal extra damage each turn via Sneak Attack, which is great when combined with Cunning Action: Hide to gain Advantage.
The addition of a warrior class brings much-needed bulk to the character in the form of more hit points and better armor/weapon options. The Fighter will give free healing via Second Wind, access to spells if Eldritch Knight is chosen, access to more combat abilities if Battle Master is chosen, and, more importantly, Action Surge, giving the character some much-needed extra Actions in combat.
The Barbarian is another great option, as they can make for a great Dex-build when wearing no armor, Rage gives resistance to weapon damage, while Wildheart gives access to tons of useful abilities. Barbarians also have the highest Hit Dice in the game, giving you some badly needed hit points.
A Paladin With Healing Abilities
Approaching Baldur’s Gate 3 with a high Charisma score is the ideal way to avoid combat. There are many battles that can be avoided with a good Deception or Persuasion roll, convincing enemies to back down. The best warrior class suited for this is the Paladin, as the Charisma stat is used for many of their abilities.
Paladins are natural tanks with all armor & weapon proficiencies, high Hit Dice, and the ability to heal themselves with their special abilities. Thanks to their spells and Divine Smite ability, they also have incredible damage potential. All of these elements combined give them an incredible spread of features for a solo run.
A Beefy Druid Who Uses Wild Shapes & Spells
The three healing spellcaster classes (Bard, Cleric, and Druid) all have benefits regarding solo runs. The Druid comes out ahead, mostly due to their Wild Shape ability. When a Druid uses Wild Shape, they essentially gain a second health bar and a whole new selection of attacks. This means they have a great spread of abilities across the animal forms, giving them a great toolset for dealing with different foes.
Along with Wild Shape, Druids are primary spellcasters with access to powerful healing, offensive, and summoning spells. This gives them a ton of options for spending their spell slots. They also gain some amazing Cantrips, like Thorn Whip, which is great for manipulating enemy movement, and Produce Flame, which acts as a light source, attack spell, and can ignite surfaces, giving it a ton of utility.