User blog comment:Gruntburner/Mass Effect 3 Achievements Revealed/@comment-4118072-20120204224838/@comment-3228423-20120208180456

@Ironreaper: I said it was a possibility that the achievement was talking about him. Besides, even if (didn't read the books, so I'm going with if) it was never mentioned that they trained together, Kai Leng's page says that he's a former N7, which may have meant that they did. And a possible explaination as to why it was never mentioned was because Shepard was sickened by Kai Leng's xenophobic behavior so much that Shepard never wanted to mention Kai Leng again. Possibly the same the other way around.

Who knows, the "Old Adversery" mentioned in the achievement may mean Cerberus itself.