Mission 88 - Dismantle the Airdrop Operations
Pull up to the objective marker, then hop the fence to your left, making your way around the north end of the building. Vault over a little concrete barrier to enter the compound proper.
Snipe or sneak your way to the main building. There are a fair number of hostiles, but they're so far apart that even if the alarm is raised, you'll like never be facing more than one at a time.
Enter the building from the external stairs- once inside you should have a good angle on the informant. Cripple him now for safekeeping, then finish off nearby hostiles before putting him to the knife.
This building has a ton of goodies here- cash, guns, and armor, so fill your pockets before heading out to the marina.
You're going end up doing a bit of fortress storming for this next bit anyway, so might as well enter through the front parking lot. You'll want these guys dead so they don't come running at your back if things blow up later.
There are Mass Effect:2 levels of hip-height cover in the next area, so just advance and kill as silently as possible. Ignore the tempting fuel tank by the boat- the boat will leave shortly, and you can off the remaining goon as normal.
Watch the corners as you approach the building- there's a sniper on the roof, and some patrollers inside. But they don't communicate well, so move through them to the upper office, and take out Butterbean before he even knows you're there.
He's got a nice rifle (Mayweather M04A3) if you want to trade up, otherwise it's back to Pablo to wrap up this racket.
It's hard to imagine you've killed Butterbean and not hit the damage limit, but just in case, there a few crates here you can snipe as well.