The best entrance into the JD Trumbet's Seaside Restaurant is via the parking lot to the west. If you want to snipe the area you can take position from the western underpass, or on top of it, but I like to get in close. Use the cars for cover and bring up the Protector to draw in enemies to take them down, but be careful of the guy on the second floor balcony. Once the two men in the parking lot are dealt with you can head up to the second floor, where you will have an advantage over the rest of the enemies with plenty of environmental objects to hack.
Stealth Route - Getting Your Hands Dirty
The best route to take for a stealth run is to head in from the west parking lot, taking out the enemies on the ground first.
You can then move to the southern area by the rear of the building and wait patiently for a guard to come close enough to takedown, followed by his buddy who likes to investigate. You can then head counterclockwise around the building to the beach and take out another guard before heading up to the second level. There are only a few more enemies to deal with now, so take them out quickly.
Aggressive Route - Show 'em What You Got
By now you should be pretty good at aggressive tactics, especially if you got a Spec-Ops gun early. Use the environment to kill enemies or draw them into your line of sight and create a blood bath as you take out the nine men.
Whichever path you choose, make sure to be ready for reinforcements that arrive in the parking lot, one of which is an Enforcer. If you can, kill the final guard by the parking lot to give you a good view of the incoming enemies that is perfect. If not, you will want to be on the second floor so you have a height advantage. Optionally, you can place Proximity IED's in the parking lot near the other cars so that when the enemies arrive they are taken out without worry.
Once all the reinforcements are down your objective will update and you can rescue Jackson in the security room at the top of the building to complete the mission.