Usurpation

Usurpation is awarded for defeating the Cultist King for the first time, and despite the boss-fight framing it carries a surprisingly gentle 1 difficulty star while paying 4 diamonds. That low rating reflects that the King is a scripted encounter you can prepare for, not a random ambush, so a well-equipped player can take him down reliably.

The key is preparation before you trigger the fight. Show up with upgraded weapons and a stack of healing, because the Cultist King hits harder than the regular cultists you have been clearing. Clear his minions or other adds first so you are not fighting on multiple fronts, then focus your damage on the King himself and keep moving to avoid his heavy attacks.

Bringing a teammate or two turns this from a careful solo duel into a straightforward gangup, since you can split his attention and revive each other if things go sideways. Because the badge only needs the first kill, there is no streak or repeat requirement, so one clean, well-prepared run is all it takes to bank these 4 diamonds and unlock the achievement.