Ideally unions should start creating defences and blocking entrance wherever they show up.
Through personal action maybe it would be more effective to try and lure them or sabotage them creatively. Give them false directions, pretend you want to take part in it and waste their time, lock any doors you have keys to. Talk with any immigrants who would be targeted by such an attack before it happens and make a plan so they are prepared to use that time window to escape.