The skill requirement for Titans in the Crucible is pretty high right now. Once you become really good at the Striker subclass, you can really do a lot of damage. Just abuse the shit out of the shoulder charge, your shotgun and pulse grenades. Take a good, precision primary weapon to handle threats beyond your shotgun's reach and try to prematurely charge your shoulder charge before you rush enemies in a defensive position (around a control point, salvage point, etc.) Stick to confined quarters as that is where your abilities are most useful.