Just did a DLC Versus match. It's all right; the challenge definitely doesn't come from shooting other players, but surviving the onslaught of Manjini
WHILE shooting/avoiding the other players' attacks. I'm not sure if there's a connection with Mercenaries for stages, but I had everything open from the start. Only Chris and Sheva are available initially; you don't have to do anything to unlock the other characters though, instead they are now unlockables that must be purchased using points earned from playing any of the modes. The most expensive characters were 30,000 and 50,000 points each.

Gonna take a while.
Did one Survival match (four players; every man for himself with Manjini) and got first place. You start with over seven minutes on the clock and some more time can be found, as can a few bombs/weapons. You get points only by killing other players, so the Manjini are just obstacles it seems. It was easy enough to keep the lead after I chucked some flame grenades at two of the more clueless players and then just ran and avoided them until the time ran out.

You have unlimited lives, but lose points if you die. I can see this being fun for a little while; glad they made it 400 points and not 800 points; don't know if it would have been worth that.
Versus mode appears to not be an unlock key for once, BTW. Although the file is only 1.86mb, they are just re-using all the game assets again.