Running the mission on a dedicated server and it seems the AI are nigh-on invincible. It takes three mag dumps from a rifle calibre weapon to down them, and even then they may not stay down and then they take 6 or so headshots to fully kill.

We do have several mods including AI altering ones. (VCOM & LAMBS) although i've played with these mods enabled before and it's not been this much of an issue.