You need to have a group or ally/boss/hostage who is restricted to the required range.
The range of a mission is the intersection of level spreads for all groups/specified mobs within the mission. And custom mobs count as 1-54.
So if you want to lockdown a mission with rikti as the default group to 30-38 you could try adding a group of prisoners (range 30-38) at some point. And you can do this in many cases so it's not easily noticeable.
It's due an update to a revised version (though RL issues have stopped me doing this so far) but see Notes on Groups and Spawns - part 5 has some notes on using groups for locking down level ranges.
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Very helpfull, thanks alot. 
Didn´t realize the level of the hostages counts towards the minimum level of the missions. Now I just need to find a level 20 hostage for my arc, shouldn´t be to hard.
Thanks again,
Formaz
Hi everyone.
Yesterday I saw a bunch of missions with mobs in it that started at a very high level. I remember one specificly with Rikti level 52-52. I tried to duplicate that and ran into the problem that I dunno how I can alter the minimum level of mobs in missions. The highest Rikti I found start at level 46. How can you change that?
More importantly, can you do that with a custom group as well? Cause I created an arc that should start at level 20 with my custom group. However, I don´t find the switch that lets me determine the level of my mobs.
Anyone knows if something like that is doable?
Thanks in advance,
Formaz