开火 is a separable verb (离合词) meaning 'to open fire' or 'to start shooting'. The verb 开 and the object 火 can split apart when you insert aspect markers (了, 过), durations, or measure words between them — for example, 开了火 ('opened fire'), 开过火 ('have opened fire before'), 开了一次火 ('opened fire once'). You cannot put a direct object after 开火; the target is introduced with 向 or 对 (向敌人开火 'open fire at the enemy').