If you use guren on a pack and hit 2 targets with it, you basically got 850x2 potency out of its 2 minute cooldown, instead of the single hit you would have got on the boss. If you think about it, that's kinda like getting to use it twice, so there's your '60s cooldown ability'.

And why stop there? If you hit 4 targets with it, it basically has a '30s cooldown', hit 8 targets and you get so much bang for your buck it's like it has a 15s cooldown. Finding more targets to abuse aoe skills is the best way to make use of them, you're basically cheating the game into letting you 'use' them multiple times at once.