maybe I'm remembering things wrong and it was two Slime Gunners or a large one, all I remember was that one encounter destroyed me and I've been scared of them ever since.
I did find a counter to them though on a veteran mission with all novices, get a noble with high reflex save and have her spam "Appel," the noble I had had 18 dex and a condom mask, (I think red is reflex and the black one's fortitude,) the slimes kept trying to set a status to an enemy that could dodge, riposte and shrug it off if they did hit