I'm currently trying to get the rep of one of my chars to where I want it. To achieve that, I have to either kill a lot of NPC-Xenos, or take a bribe that would mess up more than it would fix.
To make it short: I have problems finding NPC-Xenos. I tried shooting at their bases and platforms, I tried flying around in Liberty with a full cargo-hold. I did it for quite a while now and I can count the NPC-Xenos I encountered with my fingers.
If you want to rep unlawful, take missions from LH in Ontario; you can also drop the missions before you complete them, that will give you the desired effect from the Xeno-hunting, which will equal out the loss at your LH rep so you can easily take the next mission.
Just a word of warning to you. I have noticed that even though it says that Xenos are hated by everyone, you will take small faction hits with the IMG when killing them. I actually made myself hated by the IMG just by killing Xenos.