You can't do the ambush. The closest you can get is a map with a small "back" area with only two spawn points well within aggro range of each other, and that doesn't guarantee the second boss will aggro. If he's a custom, he might be more likely to aggro if you give him a secondary with +perception.
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I searched and found a thread from a while back saying this couldn't be done, but with all the changes to the MA I thought I'd ask again, to be sure.
I'm trying to get a single boss to spawn as an ambush when the main boss of the mission gets to 1/2 health, but no luck so far. The best I can do is spawn another boss fight when the main boss gets to low health, but it's usually out of sight range and certainly doesn't run in to the rescue like I'd want. I don't see any options to make it possible, but wondered if anybody figured out any tricks to get it to work.
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