You're talking about the bridge at the end with the spike roller? When I did it coop, I had Lara grapple the ring from a good distance away. Then Totec walks up the rope, plants a bomb and walks back down. Lara has to be backpedalling here to make sure the rope doesn't rest on the spike roller. Once both characters are back on the bridge, Totec detonates the bomb to set the spike thing rolling. Plant a bomb, back away, then detonate when the roller is over the bomb. This pops it up into the air and you can roll under it to safety, victory, and everlasting glory.