In Resistance 2 co-op during the subway tunnels part of the Bracknell map, there are a number of titans that teleport in at various points. Usually these guys take a lot of punishment before they go down, especially when you get the extra-strength ones that are handed out when you have a full-up party. But sometimes I get in a party where someone seems to know how to one-shot them. The things hardly get two steps out of the teleporter and they're toast.
I've personally tried the LAARK and the pulse cannon on the beasts, and as far as I can tell, either of these would need at least four shots to take down a full-strength titan. I've seen other people trying to use these weapons, and getting the same effect I do. Whatever is going on, it seems to be someone down in front, close to the titan, that whacks it. I don't think it's combined LAARK shots b/c I can usually see most of the soldier's wraith shields up.
Could this be a map glitch? Maybe there's a sweet spot from which you can get a magic shot off? It seems to be this one part of this one map where this happens. Everywhere else there are titans, they come out and stomp around for quite a while before dying.