Yeah, kinda strange stuff, some alien inspired gangsters build more protected and large mansions, than the Counsil.
The key strategy move I see is to avoid open spaces to not be hit by gillmen, vehicles, assassins, nor be a target for explosives and heavy fire. Get into the nearest building ASAP, and capture small building one by one. After you reduce enemy quantity enough, you can leave buildings and end remain. I would recomend to take proximity grenades since there is a lot of narrow spaces and sometimes tricky enemies which is dangerous to just engage and fire at (assassins and gillmen). Maybe it's a good idea to bring some explosives to break down the mansion's wall, since it's hard to assault the main intrance, but I didn't tried it yet. It would only allow to shoot into, not to get into, there is a one level high foundation.
If you have no weapon to deal with vehicles, you can pick a machine gun or a sniper rifle from your enemies (Red Dawn has a lot of PKMs for example, it will take long with them, but it would be better, than spend BO ammo). When I tried to pick a special gun for vehicle, I used light cannons, and they was not really good: extremely low fire rate, average accuracy and barely good enough damage for armor piercing made them just slightly better than PKM. And, BTW, vehichles are vulnarable for electricity, so you can beat them with electric clubs if you catch them close.