When safe (for example, between enemy waves), use a melee weapon to save ammunition such to break objects or when only a few enemies are present. Use sniper rifles to eliminate many enemies at once. The amount of time required to set up a shot is worth the effort. Sniper rifles are also useful against Bosses. When possible, circle around a group of enemies and place your back against the left side of the screen to line up a sniper rifle shot easier. More enemies will often appear from the right side. Sack-carrying thief girls will occasionally appear between enemy waves. Equip a rapid-fire weapon, such as a pistol, to take them out quickly for large rewards.