Grandpa drops his spell on the leader and four of the skitterers, lighting the area up in a soft glowing light.

((Assuming that Tides wasn't waiting to go off, go ahead and roll a d20, surging on a 1))

Three of the skitterers are lit up with a purple glow as Larry dashes after Grandpa and Black, while Bryn sends another icy blast at one of the newly illuminated skitterers.

Attack: 25 Damage: 9

Keller: 21    <-----<<
Grandpa: 19  
Black: 19 
Larry: 15
Brynn: 15
Bolt: 11     <-----<<
Skitterers: 8 (X,X/X/-X/-9/X)
Skitterers: 4 (?/?/?/?/?)
Hulking Figure: 2 (-103) (oops, lost track... it's a lot)
BB: 2
Leader: 2 (-10)

No Tides indeed: 9 

I seem to have inspiration on my sheet that I've never used, so if possible, I'll use that to negate the disadvantage, but if not, here's the roll: 24

Keller grunts and moves 30 feet closer, and then dashes another 30 feet east with cunning action.

He aims and fires a chaos bolt at the leader, using Tides for advantage. (Tides: 30)

Attack: 21 Damage: 7+1=8 acid damage plus 4 (mostly rolled in game log for visibility of each die)

((Sorry for the delays, work's been draining lately.))

With a backswing of her axe, Bolt lops the head off of the large creature.  With the momentum of the slash, she spins and starts 'bolting' towards the rest of the fight (Action to dash)

((Yep, inspiration to make it flat works.))

Keller hurries closer to the leader, brimming with magical potential as he hurls an orb of acid at the leader. There is a shimmering, and Keller's magic slams into one of the skitterers which the leader sacrificed via reciprocal teleport, allowing the underling damage by Bryn's spell to take the brunt of it instead, the acid eating into it as it collapses.

Keller:

Bolt turns from her kill and hurries to join the battle.

The remaining skitterers swarm forward, but only one is able to make a bite towards BB.

Bite with Disadv: Attack: 19 Damage: 7

Grandpa, Larry, BB, Black and Bryn are subjected to the maddening chittering of the skitterers and feel unfocused and uneasy. 

Oops, didn't apply Blacks Disadvantage to the roll: 23

((The bite lands on BB, and he'll need to make WIS save, with disadvantage please!))

Keller: 21
Grandpa: 19  
Black: 19 
Larry: 15
Brynn: 15
Bolt: 11  
Skitterers: 8 (X,X/X/-X/-9/X)
Skitterers: 4 (X/?/?/?/?)
Hulking Figure: 2 (-103) (oops, lost track... it's a lot)
BB: 2  (DA WIS Save)  <-----<<
Leader: 2 (-55ish)

Wisdom save da: 10

BB seems befuddled, his defenses slipping.

((Attacks against BB have advantage until the skitterer's next turn.))

"Uh, woooow! I paid for this once, woooow."

BB will stubbornly misty step to the boss.

Wild magic: 7

Then attack twice, once with tides.

Attack: 15 Damage: 7

Attack: 11 Damage: 9

"Come onnnn!"

((Misunderstanding!))

 ((BB's Tides from previous Con save use triggers safety mistyv step, d100 for surge!))

((Mess up roll: 72))

BB feels himself surge with magical energy, certain he could expend additional magic if he has it.

((That'd become active when you cast misty step as a bonus action, so if BB had a spell of 1st level or higher that he wanted to cast with his action he could do so, and gain a bit of temp HP at the end of his turn. That's information you'd have before making the attacks, if you want to retract those.))

((Totally taking those back...cough.))

Thunderwave! (con save of 12 for half damage): 10

((And the healing: 3))

((Not true healing, just temp HP!))

CON: 9

The leader, anticipating a couple of horrendously off target whiffs from BB, is caught completely unprepared for a blast of thunderous magic from the fighter, and gets hurtled backwards to land in the rubble, taking 1 additional damage.