Some of the brothers are loading a van, letting them go makes things less complicated.
Killing all 10 enemies from the vantage point stealthily will greatly conserve your health and armor.
Unmarked - Complete with minimal damage to health and armor.
But if you want all of the headshots you must kill those two before they leave. Normally this just mean you have two less enemies to worry about.
If Trevor takes too long, two of the O'Neil brothers will finish loading the van and drive off.
There's two on the lower left most playing with pistols, one drinking on the left of the ground floor, one scouting and two chatting on the second floor, two chatting and two loading a van on the lower right.
Perhaps an oversight, your headshots only count for the 10 enemies who are clearly visible from the vantage point.
Headshots - Kill 10 enemies with a headshot.
Make a gas trail from the meth lab to the outside of the house.
Kill all the O'Neil brothers and reach the meth lab.
Trevor then picks up a Jerry can and pours a trail of petrol outside and all through the house, he ignites the trail, causing the house to explode in a blazing inferno. Upon arrival, Trevor begins a heavy assault against the guards and goes to the basement where the O'Neil's meth lab is located. Trevor, now furious, proceeds to the O'Neil ranch, calling to threaten Elwood O'Neil on the way there. When they mention they want to pursue other jobs, Trevor threatens them to get them to tell him who they are working with, the O'Neil brothers. Trevor arrives at the Yellow Jack Inn and confronts Tao and his interpreter.