The Wildemount Irregulars (2020)

Alton chuckles and says to the Gnolls, "I warned you about waking the dragon, but I guess you didn't Gnoll any better." He casts enlarge/reduce on Balassar using the enlarge option, and also gives him some bardic inspiration with the bad pun.

Movement: None
Action: Casts enlarge/reduce on Balassar. Until the spell ends, the target also has advantage on Strength checks and Strength saving throws; and does an extra 1d4 damage.
Bonus Action: Bardic inspiration to Balassar. For 10 minutes/combat rounds, he can add 1d8 to one ability check, attack roll, or saving throw.

Donning a shield is an action, yes. Balassar stands, readies himself for a fight, and grows bigger... and then bigger still. He's massive. Alton's spell takes effect, as his music rings in Balassar's ears, the dragonborn feels ready for anything - certainly some puny gnolls. I think Silent and Alton would both be pushed back a little by Balassar's new size - he takes up a lot of room.

Round Two:
Gnolls
Gnoll Leader
Alton, Balassar and Silent

The gnolls ears flatten, but the leader growls and snarls and they run in to attack, those not already next to Balassar.

Let's see how this goes:

Gnoll 1:
Bite Attack: 5 Damage: 4
Short sword Attack: 7 Damage: Unable to parse dice roll.
Short sword Attack: 24 Damage: 10

Gnoll 2:
Bite Attack: 6 Damage: 6
Short sword Attack: 17 Damage: 3
Short sword Attack: 5 Damage: 4

Gnoll 3:
Bite Attack: 21 Damage: 3
Short sword Attack: 13 Damage: 3
Short sword Attack: 6 Damage: 4

Gnoll 4:
Bite Attack: 15 Damage: 6
Short sword Attack: 23 Damage: 4
Short sword Attack: 15 Damage: 8

Gnoll 5:
Bite Attack: 20 Damage: 4
Short sword Attack: 16 Damage: 4
Short sword Attack: 22 Damage: 5

Between them, they do 24 points of slashing and piercing damage to Mt. Balassar. Barely a scratch, right?

Gnoll 6 skirts around the wagon and approaches Balassar from the other side; maybe looking to give some of the other gnolls an advantage next attack?

The gnoll leader carries a rusty glaive and uses it to attack Silent's Echo:
Attack: 10 Damage: 10
Attack: 20 Damage: 8

The first attack misses, the second hits. The Echo pops out of reality, and the Gnoll Leader gives a fresh snarl, this time of satisfaction.

Alton, Silent and Balassar are  up.

Silent moves 25 feet to get adjacent to both Gnoll 1 and Gnoll 2 (I don't think he should provoke any attacks of opportunity while moving to position himself here since he's pivoting around Gnoll 1 and ends diagonal to it).

He uses a bonus action to Manifest an Echo adjacent to the gnoll leader and has it take two slashes at the gnoll leader again. 

Attack 28 Damage: 26        

Attack 25 Damage: 16    

The Echo pops back into existence. Is it the same one? Or a different one? A question for the philosophers. The gnoll 's fate is less of a mystery - the echo's second attack takes its head clean off.

Alton and Balassar are up.

Alton moves around to get behind Gnoll 6 and, using the now dragon size Balassar as a distraction, tries to stab the gnoll in the back.

Movement: Behind Gnoll 6
Action: Attack's Gnoll 6 with the rapier, Attack: 24 Damage: 9 Piercing.

Balassar just smiles as he brings his blade down on the gnoll closes to the cart (Gnoll 5). He then lets out a roar as he brings his blade down on the gnoll next to it (Gnoll 4). Letting out another roar as he just looks down on the gnolls.

OOC:

Attack: 17 Damage: 8

Enlarge Damage: 3

Attack: 27 Damage: 11

Enlarge Damage: 3

Giant's Strength on whichever hits: 3

Action Surge:

Attack: 14 Damage: 10

Enlarge Damage: 3

Attack: 14 Damage: 10

Enlarge Damage: 4

Giant's Strength on whichever hits: 4

OOC:
Using bardic inspiration on that 9 for attacking if it doesn't hit: 7

Gnoll 6 takes a stab in the back from Alton, and turns to focus on him.

Two strong blows from Balassar, and Gnoll 5 goes down with a yelp. Gnoll 4 takes a single hit, and yelps a horrific, high-pitched panicked yelp as Balassar hurts it once, and then again - it's too much for the creature, and it drops to the ground, dead.

Round Three:
Gnolls
Alton, Balassar and Silent

The gnolls shift in position, and keep attacking, despite the situation looking less hopeful for them than it did a few seconds ago.  Gnoll 6 attacks Alton:
Bite Attack: 12 Damage: 6
Short sword Attack: 11 Damage: 5
Short sword Attack: 21 Damage: 8

That's a bite and a stab, both connecting. Alton, give me two constitution saves, DC 10, to keep Balassar's spell up.

Con save 1: 16
Con save 2: 13

Balasaar shrinks back down to merely large size.

Gnolls 1, 2 and 3 try to tear into him, ripping and biting and stabbing:

Gnoll 1:
Bite Attack: 17 Damage: 4
Short Sword Attack: 6 Damage: 7
Short Sword Attack: 16 Damage: 7

Gnoll 2:
Bite Attack: 22 Damage: 3
Short Sword Attack: 15 Damage: 7
Short Sword Attack: 16 Damage: 8

Gnoll 3:
Bite Attack: 11 Damage: 3
Short Sword Attack: 16 Damage: 8
Short Sword Attack: 20 Damage: 7 

I make that one successful attack, for 3 damage, right?

Alton, Balasaar and Silent are up.

Balassar lashes Back at the Gnoll that strikes him as it’s blade cuts him. He then bring his blade down on the Gnoll to the left and moves it up at an angle at the other Gnoll to his right.

He then takes a deep breath waiting for what else is to come.

OOC:

Reaction:Vengeful Assault against Gnoll 2: Attack: 12 Damage: 7 

Attack on Gnoll 122 Damage: 10

Attack on Gnoll 3: 26 Damage: 10

Giant's Strength on whichever hits first: 1

second Breath: 7

Gnoll 1 and 3 take hits - Gnoll 1 a bad one. But the vengeful assault doesn't hit.

Silent and Alton are up.

Alton whips his rapier around trying to carve the letter A onto an exposed part of gnoll 6. "Gnoll doubt about it, Silent. These guys picked the wrong camp to ambush." 

Action: Attack Gnoll 6 with the rapier, Attack: 16 Damage: 9 piercing. 
Bonus Action: Gives Silent some bardic inspiration with that pun. For 10 minutes/combat rounds, Silent can add 1d8 to one ability check, attack roll, or saving throw.

Silent laughs loudly at the pun and moves 30 ft. to flank gnoll 2 with Balassar (giving gnoll 1 a wide berth as he does), finally drawing his own sword and taking two swings at the beast. 

Attack 10 Damage: Unable to parse dice roll.         

Attack 17 Damage: 14