Crassus has a Passive Investigation of 11, so he doesn't even need to roll.

The Crassuses reasoning is as follows: he notices the fire was placed strategically to drive them towards the second exit. The chance that bandits haven't noticed it are slim, since it is, as they claimed, their territory. Therefore, the untouched door is probably covered heavily, whereas the front door is likely to have less people watching it. The size of the barn and the roar of fire will prevent people from hearing the battle on either side of it, so reinforcements will hardly be an issue.

To reach the front door and get out one would need to put out fires blocking exit. Thankfully, the druid got this covered.

Crassus, having some experience with battle tactics and leading enemies into a trap is able to clearly see that these bandits didn't leave the back door untouched because they were being nice. He's able to reason that the bandits are waiting right outside ready to kill him and his companions and will advise his party not to go out the back as Pict puts out the fire.

"Those lying bastards are right outside the back. Don't go out that way."

Violet will hastily get hold of her important belongings (at least her backpack) and head out of the front door coughing but furious with anger "why couldn't they listen, why can't anyone ever LISTEN"

Erlan 

I will follow my way behind Crassus. My oath bow readied with an arrow nocked. “You better be right Dragonborn.” 

Pict is still startled from waking up to the blazing fire around them, and is on edge. He slides his hand across his quarterstaff and casts shillelagh before following the party and stepping outside.

"Be wary friends."

Crassuses plan is solid. The group gets out of a flaming barn, warded from fire by the summoned water. There's some space in front of the barn with three people in front of it. Berserker and Bandit Captain you've previously met and the unknown Archer. The berserker is the closest to you, captain stands a bit behind and archer is farther still.

"I knew you'd figure it out, you mind-reading weirdos!" Captin yells. "Attack!"

Okay, battle time again. The same rules apply. I will make initiative rolls on behalf of players using your initiative (Sarcasticfury, you may re-roll with your dice, I know you prefer it that way).

Archer is standing 45 ft. away, you won't be able to reach him in melee in one action.

Violet, false life is fine, cast away and add to your dice before beginng of the next action.

The sound of the fire is loud enough so that the rest of bandits won't hear the battle, but using loud spell like Thunderclap will attract their attention.

Initiative rolls:

Crassus: 13

Violet: 6

Erlan: 16

Pict: 2

Bandit Captain: 6

Berserker: 18

Archer: 15

Okay, so we have our order.

1. Berserker
2. Erlan
3. Archer
4. Crassus
5. Violet
6. Captain
7. Pict

Berserker roars at the command and charges forward, focusing his attack on Crassus. The fighters loch eyes and ready their weapons. Berserker swing the greataxe at the Dragonborn, attempting to crush his armor in one reckless poweful motion.

To hit: 12against AC of 16

Damage: 11

Due to a berserker reckless feat, he rolls with advantage to attack, so all attacks against him also have advantage untill start of the next turn.

Erlan

I take the archer in my sights behind my oathbow as I utter the command word. With the bow string taught, I release an arrow toward my ranger enemy. 

Attack: 20 Adv: 10

Damage: 7 Sworn enemy damage:  7

Bandit Captain lets out a scowl when hit by the arrows.

"Don't just stand there, shoot him!", he yells at the archer. The archer listens.

The archer has multiattack and therefore can make two attacks with a longbow, both of which he makes agains Erlan.

Ranged attack: 17; 15

Damage: 8; 10

As a bonus action, the enemy uses Archer's Eye, adding 4 to his next damage roll.

Crassus is going to take a swing at the Berserker that missed him.

Attack: 14

Attack with Advantage: 16

Damage: 9

Crassus will also spend a Superiority Die to trip, which will add 8 points of damage, bringing the total to 17 points of damage, and the Berserker has to make a DC 13 Strength Save (I rolled a natural 5 for him, which I assume fails.)

Violet focuses her attention on the Bandit Captain. You can hear her arcane summoning of mystical powers. "Maledizione" a mass of energy forms inside her hand and is hurled at the captain "Forza Elementare"

The Bandit captain is engulfed in lightning

Bandit captain screams when the the warping ball of pure chaos hits him in the middle of the chest. He screams even louder and drops to his knees when lightning convulses his muscles, but it takes so much more to take him down. Captain turns his attention to the witch, pulls down his daggers and throws them.

Due to multiattack, he can make 2 ranged weapon attacks or 3 melee ones. Violets AC is 12

Dagger throw: 14; 17.

Damage: 5; 7

Pict observes the battle unfolding with growing concern, seeing his allies take several serious blows from the enemy.

"Stay clear, lad!" He yells to Crassus, and he calls upon a new power, casting moonbeam on the Berserker that Crassus is engaged with.

Okay, Pict, you forgot to roll. Allow me: saving throw 7

Callindra, you're right, disregard the first dagger.

Saving throw fails, berserker takes 14 radiant damage.

So far we have: Berserker with 36 HP, Bandit Captain with 55 HP and Archer with 61 HP.

Your HP are in your character sheets.

Berserker decides to take another swing at the fighter.

Attack (AC is 16): 19

Damage: 14

Attacks against berserker have advantage for the next turn.