Do not sprint towards crowded areas when a match begins. Instead, wait momentarily. Although you will reach the battle slightly later, you will have an easier time to kill enemies that were softened up. Consider trying this in training matches. Try to move and aim at the same time. Although this can be difficult, you will be able to take out stationary enemies easier after they begin firing. When many enemies are shooting at one another, shoot everybody. Even if they are not killed, you will earn assist bonus points.