Those nasty swarms are actually pretty manageable once you understand their limitations (they're actually designed to teach you about vertical positioning). The first thing you'll want to do is get yourself up on any elevated surface you can find - boxes, pipes, even broken walls work great, because these mutants can't climb at all (this is actually a behavior shared with several other mutant types you'll encounter later).

From up there you've got a perfect spot to handle the situation, and grenades are your best friend here. Think of it like crowd control in classic arena shooters - you want to wait until they cluster up, which they naturally do when trying to reach you. This makes your explosives way more effective since these swarms tend to bunch up right beneath elevated positions (pretty clever ai behavior when you think about it).

A good tip here is to always keep at least two grenades specifically for these encounters, since their area damage is much more efficient than trying to shoot individual rats. Once you've thinned them out with explosives, any stragglers can be picked off safely with basic weapons, but don't get overconfident and drop down too early - these things can overwhelm you in seconds if you lose your height advantage.