There really isn't a weak spot that I know of.
If you are talking about co-op, you really have to have teamwork to bring them down (especially on super difficulty mode). You need at least 1 of each class usually. The soldier's shields protect the team, the recon class keeps everyone with ammo, and the medic will keep everyone with health.
In the single player, I've used the bullseye, which allows you to mark a target. Mark the titan and then blast away, every shot will hit. On normal do that 3 or 4 times and he'll go down.
You might want to try the bullseye trap as well.