I'm trying to defeat the remaining henchmen in "Strange Visitors," the mission where Mentor is introduced.

There's two thugs with guns and one guy with bat near the front entrance guardhouse. In the middle is a thug with gun and a thug with bat. Finally, near a side guardhouse is a thug with gun and a guy with bat.

How do I defeat them all without raising alarms?

without raising the alarm I have no clue PM me if you find out

First of all, it doesn't matter if you raise the alarm. Second of all, you can jump from building to building.

I beat it without raising alarms quite easily accually. the trick is to pick up object that weigh more than 200 pounds and use them to kill the guards. Once you get an object, pick it up, to to the roof and throw them on unsuspecting guard's heads. If you knock the guard out on the first throw, the guard standing beside him will not notice but if you don't KO him on the first throw, he will raise the alarm. Note: save alote just incase if you don't KO a guard on the first throw.

You can't complete both secondary objectives in this mission, because in order to knock out all thr thugs you need to also get the ones inside the guardhouses, which can't be done unless someone raises the alarm (unless you destroyed the guardhouses after beating all the other thugs I guess), but basically, it's a choice between one or the other.