This is the fourth mission given to you by Ray. You unlock his missions whilst completing missions for Packie, so if you have the 'Full Exploration' achievement you will have these unlocked.
The first three missions are quite easy, but this one is rather tricky. Make sure you have a gun, it's a good idea to get some body armor too.
Begin by shooting all the guys on the top level of the museum. With a bit of practice in free aim with a pistol, headshots (1-hit kills) are easily achieveable. Before heading down the stairs, grab the health on the wall next to the opening where the stairs are.
Now clear out the guys on the bottom floor, there's some armor on the counter to the left as you come down the stairs.
Go through the door, kill a few more people and quickly grab a car before Issac's guys shoot you.
Escape by driving quickly to the other end of the park and this achievement is yours.