This is merely having fun at the expense of a temporary bug which, although we might enjoy it, is not something the Admins may want to preserve in Combine lore by having to explain every six months with a series of answers that run something along the lines of:
"No, that Achievement was just available for two weeks in Year 18."
"Why? 'cause it was a bug, that's why."
"Are we gonna re-enable the bug so you can shoot a zombie Vornskr? Not on your life. We have more important things to do."
"I don't care if Rumble Stiltskin has an 'I Shot a Zombie' achievement badge and you don't!"
Yeah, that'll get old real fast. Still, live in the moment.
Fyi should be fixed. If it isn't fixed, you might need to either contact asims and get them to fix your NPCs fire delay or movement (they will need the NPC squad leaders ID), or disband and recreate your squad should also work if you are in a position to do that.
"Clarr's a big mean jerk and he steals my lunch money and makes me cry."