Gotham Knights

This achievement is won by completing a fight against Clayface in the Clayface villian questline. The first portion of this is by defeating 5 faceless muggers within the streets of Gotham. They are identified on the map by a small orange triangle as opposed to the small white triangle. After these samples you'll have to launch the mission from the Belfrey. The mission is set for a recommended level of 10-13 but I have found the actual bosses to be tougher than what the game recommends.

Some tips for the actual fight, since you have four characters to choose from:

The Clayface enemies work together very well and have a lot of tactics. Some will try to grab you from below, some will hit you at range, and some will have a delayed spike from below. If you see any bubbling, dodge immediately.

Clayface's health bar consists of three different Clayface's, and when they have low enough health they will be in a downed state. Other Clayface's can revive the downed ones, but this can be interrupted by a charged melee. This reviving ability only applies to the bigger enemies, not the small ones.

The fight is a three round affair, with the health bar fully recharging after a short cutscene each time. Second wave has smaller enemies and the third wave has smaller enemies which will explode at low health.

Achievement pops once you win the fight.