All of the guards are wearing body armor, making them a lot more durable than most of what you have been fighting. When going for a body shot, keep firing until they stop moving on the ground. If their red blip on the minimap goes out, then they are dead. Otherwise, assume they are still alive and able to fight back.
Michael leaves Tracey's Room and immediately ducks into cover at the top of the stairs. One of the Merryweather guards start rushing up the stairs. Shoot him dead. After that, Merryweather will lob a grenade into the house. Wait for it to explode then deal with the 2 additional guards that come through the front door.
After a short delay, a third will come from the left of the stairs, where the TV is. Deal with them. At least 3 more will be coming from the back of the house now. A few more will come from the kitchen and the door leading to the patio and the pool. Clear them all out and then head outside. Time to move onto the second round with Merryweather.