There does not have to be human opponents on the versus side for it to count.
We experimented by de-ranking our secondary slots all the way down to level 1 and then launching in versus against our rank only... It would appear that there are not a lot of level 1 people out there (no one in our recently played list on XBL), and we were able to swiftly take care of the level 1 AI opponents and got the achievement for a buddy that still needed it (while I was trying to find my last few audio logs). We had tried the trick on co-op hard, too, but the versus AI seemed easier.
Note that the AI is still a total b*tch even at level 1, but at least we didn't have to deal with the higher level 'abilities'...
EDIT: also of note is that the "Tough as nails" achievement does NOT require that you complete them all on the same character (as opposed to the story line based achievements), so if you do want to try the derank/low-level method, you can use that slot to complete just those missions that you still need for Tough as nails.
Last edited by A. Gamer; 05-23-2011 at 11:57 PM.