You really can't, yet. It's all tied into the functionality of Notoriety. I've seen many things suggested on the boards, almost always followed by a post of "no, that didn't work".
And is it bad that my first thought after reading the title was "Put them on a diet?" 
Dec out.
Notoriety? You mean the challenge level 1-5?
For Great Justice!
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How can I get only 1 boss per spawn? I heard somewhere one trick is to find a underling that you can add to the custom group as some kind of toy or pet, but I tried that and didn't see to work?
Any ideas?
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Was the underling level *-54, or did it cap at 50 or lower, and was your boss spawning at level 51 or higher? If the boss spawns higher than the maximum level of the underling, you won't get any underlings, you'll just get multiple bosses.
I will check into that, but the problem I was having was the reverse - no boss spawns, all underlings.
For Great Justice!
I am writing a mission where this guy sends makes a bunch of energy clones of himself - how I want to do that is a custom Boss character. But when I do that, they spawn too thickly, forcing any poor sod who solo's the mish to take on 2-3 bosses at once. I also want this boss to stand alone in his spawn - he is the dramatic center - I don't want lieuts and minions around him.
How can I get only 1 boss per spawn? I heard somewhere one trick is to find a underling that you can add to the custom group as some kind of toy or pet, but I tried that and didn't see to work?
Any ideas?
For Great Justice!