A large part of winning difficult missions (and especially multiplayer battles) is proper deck construction, i.e. the units you choose to take into battle. Your deck has 10 slots, so make them count.
Initially most players will rely on the units they unlock through normal campaign progression. Over time, you'll earn more units by completing special tasks, rescuing POWs, and as log-in rewards. You'll also earn medals with which to buy premium units, and some users will buy extra medals via IAP.
You generally want a deck with units that serve different purposes. You must buy a premium unit with low AP cost and fast production time to spam. Alternating between low level units, high level soldiers (Metal Slug protagonists, etc.), and vehicles or armored units during gameplay will help your team stay on the offensive without taking undue losses.
Here is a selection of affordable high-tier premium units and their medal costs, as ranked by Whoami55's Unit FAQ. Some units won't be available to purchase until you have cleared certain missions in the campaign.
Native (spammable) costs 30
Iron Iso (stationary tank) costs 30
Slugnoid (mech) costs 60
Gunner Unit (mech) costs 90
Trevor (soldier) costs 100
Ohumein-Conga (Green) (crab) costs 150
R-Shobu (helicopter) costs 200
Hover Vehicle (flying) costs 210