Killing the leader in a round of Guardian counts as progress towards the Abductor medal. I checked my progress before I entered the match (303), killed the leader twice, and saw my progress at 305 after the game was over. I'll try out the other CTL-specific medals to see if it works or not.
163/164 games completed (including DLC)
Completion percentage: 100%
Latest retail completion: Dragon Age II
Latest arcade completion: Dust: An Elysian Tail
Currently Ranked #1 on Achievement Completion Percentage Leaderboard on TA