Ninja and Machinist are both guaranteed to get use out of any Spear you toss their way. Summoner's a good pick too, although the reduction isn't quite as noticeable as their off-GCDs already have short cooldowns. (If you can fit it in before their Aetherflow, though, it's amazing.) Tossing one at a tank before they pop mitigation for a tank buster can also be helpful. Usually, though, I'll either eat it for a shorter Luminiferous cooldown, or Royal Road it.