you can break them into waves. all you have to do is change the trigger to send the next group when the first group is defeated. at least the last time i knew it was this way.
No you can't use Ambush Completion as a trigger. I read about this because someone had done something clever earlier but the arhcitecht won't allow it currently
Add a timer objective which automatically completes after a minute or two. This event would auto-complete and could be referenced by other triggers. If this was in place players could create their own horde mode missions.
1) Timer would link to another timer and an escort mission with no enemies
2) The escort mission with no enemies would link to an ambush
3) The second timer would be linked to a third timer and an escort mission with no enemies just like #1
This would allow people to create waves that gets tougher over time by changing the levels and numbers of ambushes firing off from the trigger.
Ambush 1 could be easy at 60 second
Ambush 2 could be medium at 120 seconds
Ambush 3 could be hard at 180 seconds
Ambush 4 and 5 would both be easy and spawn at 240 seconds
and so on until the last Ambush would be 2 simultaneous hard ambushes.
This is something you can already do with the Mission Architect but with no delay. So I was able to create 5 simultaneouse ambushes in a test mission. I need thr timer delay to break them all into waves.