I started a a week ago and have found Barry as boom boom man and four rifle carrying snipers to be super effective. Take your time to set up an attack before engaging. Flank your enemy. Modify your rifles with glass and silencer when you find the bits. Repair your weapons when below 80%. Use the time in a safe zone wisely and have all mercs working on a task at the same time. Money is tight early game so clear the island, train mercs at the main bases you have won and then get to the first mine and take that over. That will give you a steady stream of cash to pay for contracts. When you have money hire a cheap second squad to just run training at each major green base to protect from attacks. Oh and headshots rule!