Instructions
Get as close as you can to Gurangatch without setting off his radar, then hold still. He should turn away from you.
Move parallel to his motions in a stealthy manner. This may take you a couple of tries to do successfully. When you get the opportunity, launch a preemptive strike.
Have Vanille cast a round of debuffs, which allows Fang and Lightning to do more damage, since Gurangatch should be staggered. You should be able to beat the target time of 1:12 with this strategy.