Well, my first thought is that the miners probably know a few things about the plant that the guards don't; could they, say, cut the power, lock the doors to rooms containing guards (or the armoury, or if it's night the barracks), or otherwise make it so they have more control over when and where they have to face the guards?

Assault one section at a time, so the guards are scattered and can't pass a warning to others? Or attack simultaneously from different directions, so the guards are unable to meet in large numbers and reinforce each other?

Begin with hacking/sabotage rather than outright attack, so that some guards evacuate thinking there's been a real chemical spill (or radiation leak, or structural collapse, whatever)?