heh, I have tried several things with the titan.
If you find the entrance where it exits, run to the side of it before it leaves, dont letem see you, then frag the heck out of any extending arms or whatever, he wont come out.
The central pillar is right, to block boulder shots, strafing is the key, the titan's left arm is used to sling the boulders, keep it blocked, the titan will be hitable.
I had the stinger loaded and fired continuously till it ran out then used the eightball, I fragged the titan in record time that way, I was amazed.
Otherwise its a strafe/dodge the boulders game, the eightball is the best weapon, but use any available.