A good drill I often use is I set up two cones 1.5m apart and have a defender shadowing a attacker. The attacker must stay in inside and inline with the cones. The defender must keep shadowing the player while the attacker moves side to side. Have another person a few meters in front of them and have them hold up numbers on they're fingers, the defender must call the number the person is holding up, when it changes the defender must call within 3 seconds. This teaches them to keep their eyes up and defend at the same time. Hope it helps!