This achievement requires careful gameplay. The Warchiefs are the leaders of Sauron's army who stand on the podiums at the top - the aim is to
kill all 5 of these before another Uruk takes their place.
Uruks' are promoted around the army by taking vacant spaces or challenging (through some means) another Uruk and killing them. These movements take place in the following instances:
* You are killed outside of a mission whilst wandering Mordor.
* You "Advance Time" at a forge tower.
* You complete a mission aiding promotion of a high-ranking Uruk
OR you die trying to complete a power struggle mission which may lead to their promotion.
You can use the story of the game to help you here. Once you are tasked to kill 4 Warchiefs, the easiest thing to do is advance only with the main story and not complete any of the side missions, thus avoiding the risk another Uruk will be promoted (see above). The spoiler below may contain story spoilers, but will explain the point at which you will obtain the achievement in more detail.
*** Spoiler - click to reveal ***Once you get Ratbag promoted, you are tasked with killing an additional 4 of the Warchiefs. You will then have to undertake a mission to blow up the statue at the main gate, which will bring the attention of the black hand down upon Mordor. At the start of this mission Ratbag will be killed, which will remove him from the Warchief post and, provided all other Warchiefs are dead, give you the achievement after the cutscene in which he dies.