ponned89 not sure if you ever figured it out, but in server-coop.cfg you would just add one line of a command from Yaf3li's comment above (difficultyeasy, difficultyhard, difficultyfu...) and makes sure you add this in any area under other commands in the cfg like the g_speed, g_gravity command lines, thats where i put it for difficultyfu for vet and it works. Do not put a "set" in front of it, just one line being the command and thats it.
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AscensionUSA
@AscensionUSA