"Go outside and finish off the attackers" is an objective in the story mission The Third Way in Grand Theft Auto V.
Walkthrough
Rush outside as either Trevor or Michael. This is everything that they can throw at the crew. Be ready with some heavy artillery. There are a few vans that will be coming in at you. The quicker you can take them out, the quicker and easier the overall fight will be. Use Grenades, Sticky Bombs, the RPG Launcher, whatever you can to deal with them all quickly.
Once outside, it is important to listen to your team. If someone is having trouble, change over to them immediately. Otherwise, work on sweeping around the outside of the foundry as whoever to deal with the various FIB Agents and Merryweather Security Guards that are in the area.
As you make your way South, be on guard. There is a Helicopter that will sweep in from that direction above you. Quickly take it out as it is a Merryweather number and has rockets. After that you have 2 Merryweather Trucks that make their appearance with a number of soldiers inside them. Quickly deal with them like the ones out front.
