Here are some tips for getting through challenging levels:
1) On the base levels, an engineer can repair buildings so either upgrade your barracks and train one or play as the engineer class. For heavy combat situations, playing as the engineer will give you the better chance of success. An engineer automatically repairs buildings by standing next to them.
2) Playing as the medic will allow you to heal your allies and prolong their life. Medics automatically heal teammates when standing close to them.
3) Your allies will follow you so between rounds or during moments when the enemy is regrouping, you can move to your medtent. Their health will be restored as they get near the tent.
4) While rocket troopers only target buildings and do the most damage to them, gunfire from all enemies will damage buildings. If you stand near a building, that building will take damage from stray bullets so be wary about drawing fire around heavily damaged buildings.
5) While playing as the engineer, standing next to a med tent is a brutally effective defense. The med tent heals the engineer and the engineer repairs the med tent. This is only effective when the enemy is advancing from a position with a line of sight from the med tent.
6) Be careful about spending too much money in early stages on the base levels. If you upgrade unnecessarily, you may find yourself short on cash in the later levels.
