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.
135/136 games completed (including DLC)
Completion percentage: 99.40%
Latest retail completion: Earth Defense Force IA
Latest arcade completion: Warface