DM Academy

This might help with him rolling poorly. I run a swarms AC differently than other creatures. I wanted to give some the peoper feel of being everywhere around the party and settled on this as how to simulate it. When it's at full HP the swarm has a much lower AC, I usually subtract 5 from its stat block. This is because you can barely swing a sword without hitting them, theres just so many. Once the swarm has lost half of its HP I add 5 to the AC on the stat block, because as they thin out they get harder to hit.

Obviously this doesn't work in every situation. To keep combat from grinding to a halt this works best when the swarms are part of an encounter with other creatures. I used it as a supplement to a higher creature’s fight and it kept them relevant as a distraction during the fight. If it's just the swarms and the party struggles to finish them off don't forget that they can retreat under such loses. You don't want them to just keep swinging and missing if they're really struggling.

One Roll Engine has a really cool mechanic for this. They're called "unworthy opponents". Here's basically how it works:

Each UO has one stat: its Threat. This is 1 to 4. Any hit whose damage beats their Threat takes them out. They all move together and they all roll at the same time. The ORE takes sets from their massed roll to sum up the whole group's hits, blocks, and parries. Each group of UO rolls a number of dice equal to their total Threat, max 15d10 per group. At a moment's notice you know where they hit you, how many times, how hard, and whatever else. You can drop them like flies or they can overwhelm you by sheer numbers. It's super cinematic and takes little time. There's a bit more for buffing, debuffing, scaring them off, and stuff like that but you get the gist.

Maybe there's a way to capture some of that magic in your system of preference. Hopefully it at least gives you some of your own ideas. Best of luck!

I love running hordes of monsters. Here are some guidelines I offer: https://slyflourish.com/running_hordes.html