Use the right troops in missions.

Each unit has a type assigned to them: tank, fighter, soldier, gun boat, artillery, battleship, bomber, carrier, and airship. Each unit also has three types they are strong against. When you deploy units, make sure that the ones you use are strong against the ones the enemies have.

Deploying out strong units not only makes the battles easier but net two extra perks. One, the chance of high critical attacks against their units is increased netting extra loot, but it also prevents the enemy from taking out your units in the same fashion.

Fighting by land air, and sea.

The game has three areas to fight in: by land, air and sea. The areas themselves do not give any bonuses but they do influence what unit you can bring out (land = ground troops, air = planes, sea = ships). The combat is turn based and has a sort of half screen where units even on differing areas can fight each other.

Always make sure that lost units are replaced.

Note if any units are lost, rebuild after every battle. Units left after a fight automatically heal. When a new unit is created, you can physically place it on your land or click the cancel button. Any non placed units will be put in your inventory.

Keeping your units up will save time in waiting for more units in between battles.