They act similar to the Duke of Flies. This enemy will constantly spawn regular Flies. However, when hit, he will spit out two Pooters or Fat Flies in variation. When the Swarmer is destroyed, it will spawn a Boom Fly or a Red Boom Fly.
This enemy does not shoot projectiles, so it's best to kill the flies first. If the player has considerable firepower, focusing fire on him hoping to kill the Boom Fly as soon at it spawns (and the surrounding flies with its blast radius) is also a viable strategy.
They can float across the room, which can be annoying when there are rocks blocking your shots.
It is also worth noting that Swarmers are immune to the touch-damage effect conferred by the item My Little Unicorn or the Chariot tarot card.
This enemy is the next and possibly the final stage of the original Mulligan. As seen how the flies nested inside the mulligan, they multiplied forming the Hive, and eventually took control of the hive forming the swarmer, being nothing but a flying nest of flies. It is very intentional considering the game theme of decay, how in the early chapters they seem partially "infected" by the flies, and they eventually decayed over time in the later floors.