Upgrading the Barracks allows you to train more troops. The higher the level of your Barracks, the more troop types will become available, and the more you can place in your queue. To increase the number of troops you can have, you upgrade your Rally Point. The best troop type to have early is the Spy. That unit is very cheap and targets resource buildings. It can collect resources that you use to upgrade your base. Send in a full army of spies is the best approach to handle ealry battles. If all your troops die and you lose the battle, you will still keep all the resources that were looted. That's To get a better rating, you will need better soldiers. The higher the level of the soldier, the better it performs. Use several Hornets, which target towers and have high health. The Gunner should be sent first to draw fire. When you are going for ratings, Spies are not required.