"猎取人头的蛮人" can be translated into English as "head-hunting savages" or "headhunters among the barbarians" (though the latter is a bit more literal and less commonly used in modern English; the former is more concise and natural).
In most contexts, "head-hunting savages" would be the preferred choice as it succinctly conveys the idea of barbarians who engage in the practice of taking human heads, often as a trophy or for ritualistic purposes.