You take a map and fill every spawn location with patrols, except for that boss spawn. Set the patrols to spawn when the boss objective is defeated. Replace some of the patrols with ambushes if you want. This might not work on all maps so you will have to experiment.
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...wow, that's so simple I feel stupid now, and also answers another question: I couldn't figure out about why there was a boss fight at the end of one of the mishs I remember was empty like that...heh, thx! 
Huh, that might explain some of my problems with the same issue. Hate to drop patrols but I'll try it and see.
Thanks from me too! 
I wanna know how I've seen a few of the AE mishs I've run not have any spawns until the very back of the mish? I accidentally did it once, but no clue how now... I've gotta idea for a mish that has a boss in his HQ alone and when he's attacked the mishs spawns...well, ambushes...come to his aid, but the point is to trick you into thinkin he's alone...might sound a lil weird, but sounds better in my head.
But yeah, how can you get 0 spawns in the mish, clear until the back, or what-not???
Side not: I know Tyrant's group has teh 50-54 range, but where is the 54-54 I've seen???