D&D 5E Swarm of insects (spiders) -- How do you run this monster?

[Coroc]

The spiders will need to start climbing on the person they're attacking, but make sure to describe the players attacks as knocking the spiders off carefully, and if a 1 is rolled maybe have the character's armor/clothing nicked (no actual damage).

"There is something crawling on your chainmail, wait and hold still I shall kill it with my axe."

"Ouch goddam, let us forget about this, Fizban! Cast a fly spell and nuke it from above with a meteor storm!"

 

[Coroc]

... But how do I do that?

(Also, the swarm has 22 HP and resistance to bludgeoning, piercing, and slashing damage, which is effectively 44 HP. That's nuts for its 1/2 CR, isn't it?)

Flaming oil is your friend ...

And yea, the HP reflect that simply flailing around with your weapons is not causing much trouble to the insects, which in fact should have quite a big size individually to be affected by weapons at all.