You should get nearby the Vista Mansion. The gate is closed. Move near the fighting soldiers. After a few moments the bulldozer would arrive to make an entrance.
Get inside and turn right. Avoid the fights, circle the mansion and search for the way - staircase leading inside.
Get on top and move along. To the left from the stairway you may encounter some fighting, but ignore it. Move towards the hedge.
Circle the mansion moving among the hedges. Near the gate you should spot the doors leading inside it. But watch out - from time to time some soldiers run through it.
Inside use the stairway and move to the second floor. Eliminating soldiers you find there you should get another rifle to your collection.
In one of the rooms there is an entrance to the terrace. On the terrace you can find a machine gun and mortar. Don't use them - move to the other side and jump down.
In the basements you would find a ladder. Use it to get to the Naomi's laboratory.