90% of mobs in EX roulette dungeons can be stunned by holy. As long as you're first to stun anything you will stun the entire pack for several seconds on the first stun allowing time to fix heals and stuff then you can use it again and stun for 2-3 seconds before they start resisting it. I've hit 3 stuns on some mob packs though by that time everything will probably be almost dead anyway.