Add ambushes to it of identical mobs; chances are they will have to wipe all or near all to complete the objective, becuase they wont be able to tell which mobs are the guards.
Other then that, all I can say is set it to "hard" difficulty
Thanks for the reply. Issue is still unresolved tho even with those tricks. 
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Unbelievable. You, [subject name here], must be the pride of [subject hometown here]!
They need to add a timed type thingy to the defensible object option. Similar to what you get on the terra volta reactor trial and in the croatoa mission in which you have to protect the altar.
Add this and i will add defensible objects into missions soo much more!
I'm trying to set up an defendable object that I want to be heavily guarded, but no matter what I do, the spawn surrounding it is just a few (5-6) mobs.
I've even gone so far that I'm looking at popular farm mishes and trying to replicate it to see how to affect it, but no matter what I do, the spawn just stays the same.
Last version I did (copied), I tried it with the troll cave, an ally surrounded by medium (also tried hard) and having the release trigger the defendable object and it's spawn, but still no go.
What I really would like to do, is have a defendable object that is under heavy attack, and then after the first group is defeated, have an ambush coming and then another one etc. Basically a team challenge for honing teamwork for my SG.
Any and all spelling, grammar and logic errors are intentional so this post will blend seamlessly into the Internet
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Unbelievable. You, [subject name here], must be the pride of [subject hometown here]!