To defeat Bruz and his army in Damaged in Middle Earth: Shadow of Mordor focus on killing the enemy captains first. Over time the orcs will kill each other and some of your captains may kill some enemy captains and vice versa.
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If you focus on killing the orcs first you may end up facing a number of captains at the same time at the end which can be difficult. Also avoid killing Bruz first as he is fairly easy to deal with requiring only the occasional dodges to avoid being damaged by him.
You may have to retry a few times and get lucky with captains. Remember that you can restore health by dominating orcs. I also found the skill that freezes enemies when you vault over them particularly useful.