If I remember correctly, all of the Titans in Lords of Shadow work the same way. You have to get close enough to them to climb them, and then they have highlighted weak points that you have to stab repeatedly. Once you have stabbed all of them, a cut scene will ensue and you've successfully slain the Titan. Of course, it's much easier said than done.
Butterscotch - Are you trying to beat the Titan in less than 1:30 for the time trial, or are you just trying to beat it, period? If you have a dark crystal, you can use it to take the Titan's health down from 100% to 25%, then you can get by with stabbing each rune (the weak points stewieG was talking about) once instead of multiple times. If you're going for the time trial, you'll pretty much have to have the dark crystal.
Samsim - No, I wasn't trying to beat the Titan time trial, just trying to beat him period. I don't remember using a dark crystal the last time I beat him, of course I struggled with it last time, too. Thanks for the advice.