Aspiring Heroes of Faerûn

Not really. Imagine if someone was choking you with a cord around your neck. Trying to cut it with a knife wouldn't be very feasible, since you'd be likely to cut your own neck. Your best choice would be trying to fight and wrestle your way free of the choke hold. 

Kess acrobatics to excape! 13

(Sorry, I remember reading that she had been grappled, but I forgot about it!)

Under the threat of being strangled by this horrid web spinning humanoid. Kess kicks, wriggles and fights with desperate ferocity, and finally breaks the choke hold. Spinning around to face the creature once again, she immediately finds herself on the defensive, as it leaps forward with venom dripping from its jaws. 

Attack: 12 Damage: 10

Fortunately, Kess is too quick and successfully dodges the attack.

Vesper is up.

Three more magic missiles at the spider humanoid

12

Vesper's trio of magic missiles slam into the creature with another blast of light and energy, and you can tell that by now the thing is really starting to feel the effects of battle.

A few feet away, the centipedes continue to crawl in mass all over Lumen's body, working their way under his armor and clothing, and he feels the small, but extremely painful stinging bites of hundred of pincers. Attack: 18 Damage: 9

At the same time, the small number of spiders that escaped being crushed by Due Process continue to crawl all over Chrys and relentlessly bite at her with their own tiny jaws. Attack: 5 Damage: 4. However, there are too few of them left to cause her any lasting pain.

Top of Round 3: Kess, Lumen, Chyrs and Krackle are up.

Finally done with the spiders, Chrys moves over to support Kess and get rid of the creature attacking her.  "I've had it with bugs!  D'ya know how itchy I'm gonna be tonight?!" she says, bringing her hammer around.

Attack: 12
Damage: 8

Dice roller edit

Enkida, you can go ahead and roll action for Kess, as well.

(What is Kess's HP looking like?  If she's at 10 or under she'll BA Second Wind which would be 7 back (negating the second attack).  Ignore if her health is higher than that.)

Kess attacks with her scimitars reflexively, just as much offense as defense:
Attack 1: Attack: 23 Damage: 6
Attack 2: Attack: 15 Damage: 1

(Dice roller, that's just wrong.)

(Kess is only down 4 hp. Currently 15/19.)

Whether she's a little shaken from nearly being choked to death, or if the spider creature is still a little faster than she thought, Kess is unable to connect with either of her attacks. However, Chrys lands a fairy substantial blow on the creature's back, which thuds with a resounding crunch. The thing lets out a horrid squeal of agony as it continues to try and defend itself.

I'm guessing Lumen had no luck with any of his attempts to kill the centipedes crawling all over him with his rolling or prestidigitation or fire last turn? He's hurting.

If there's something obvious to do here mechanically, I'm a bit lost. I guess he tries to attack them with daggers by scraping them down his body, seeing Chrys evidently have success here? Unless there's a pool he can cannonball into.

Dagger attacks:

Attack: 9 Damage: 5
Offhand Attack: 22 Damage: 4

(I treated his rolling on the ground as an unarmed attack. It did some damage, but not much.)

After having little success with his last attempt to rid himself of the centipedes, Lumen finds that using his daggers to try and scrape them is a much better strategy, although he seems to have a little trouble getting the hang of it at first. After a few frantic scrapes, it looks like he’s killed about a third to half of the swarm.

Krackle is up. At this point, the spider humanoid is looking very injured, and Lumen still has a number of centipedes crawling all over him.

Krackle would look to the spider humanoid and then to the creatures crawling on lumen. Another ball of flame would form in his hand as his small head looks between the different tagets. He would then chuck the flame towards the humanoid creature once more.

Attack: 19 Damage: 2

Krackle's ball of flame strikes the spider humanoid right in the center of the abdomen, causing a small blast of fire and smoke that startles the already wounded creature. It responds with another horrid squeal of pain and then makes its move by retreating (disengaging) from the fight and running off into the trees away from the clearing, obviously hoping to save its own life.

All that's left are the rest of the centipedes that are still crawling on Lumen. Vesper is up.

Would Vesper be able to toss a Fire Bolt at the centipedes without hitting Lumen?

You can try. I'll say that you can attack with advantage, and if you hit, you will do damage to the centipedes, with minimal damage to Lumen. (1 hp) If you miss, then he will need to make a DEX save against your spell DC or take half damage from your attack.

Attack: 22 Damage: 2

With extremely careful aim and intense focus, and a reassuring glance at her companion, Vesper speaks the words of her cantrip and sends a fire bolt right towards Lumen, aiming for the his middle torso, where most of the centipedes are concentrated. Even though he's ready to jump out the way if needed, Lumen stands his ground as the arcane bolt slams into his chest. It's not her most powerful bolt, but it's enough to kill a few of the centipedes and slightly reduce their numbers before fizzling out. However, a number of them still remain and they continue to harass and bite at him with relentless annoyance. 

Attack: 7 Damage: 4

Ok, since the spider humanoid has begun to flee, everyone can take another turn now.

Krackle would launch another ball of flame towards the fleeing spider man!

Attack: 24 Damage: 9