Quoted from TomGWI:Multikills are represented by the weapons on the playfield (hammer, knife, sword, etc). They are ready when blinking. The shots are the yellow circles. You get credit for a multikill when you hit one of the yellow circles.
What I don't get is you never seem to complete them unless your ball drains. Therefore, if you are on ball 3, it seems you can't get to horde. Can anyone confirm this?
You do not have to complete the Multikill. You just need to collect it.
When the weapon is blinking, the shot is "alive" and completing a multi kill shot, the yellow dot, will earn you a kill with the blinking weapon. The weapon is still collected and already counts towards Horde progression.
Once your ball ends, the weapon insert light will go solid. This just means that the multi kill shots will not count towards the weapon that has a solid light. It also means that when you do earn a multi kill, it will not choose that weapon a second time.
Once the sixth and final multi kill is blinking, the Woodbury shot starts flashing for Horde start.
So you definitely can get Horde on ball three. Just get your final weapon blinking and the Horde shot should become available.
Marcus