Easy money and experience
Replay and farm campaign missions to build experience and money, as well as have a chance of getting more cards.
Use troops based on what you are facing. For ground troops, select a good gunner vehicle or paratroopers. Use mounted machine guns against Rocket Troopers. Use Rocket Troopers against tanks and helicopters. However, balance the different types of troops you have and do not rely solely on one specific class. Make sure you promote your troops regularly by spending cash or gold bars to increase their abilities and strength. This is especially true for ground troopers. Increasing the armor on paratroopers and rocket troopers will increase their survival time.
Start by upgrading your Supply Lines to get an early boost. Create and sacrifice low cost units to slow down the enemy if you need to strengthen your Supply Lines later. When waiting on your powerful units to finish their recharge cooldowns, upgrade your Supply Lines.