AFAIK the defence debuffs are auto-hit (you'll know if they've been hit by the streams of number 1s above their heads). 
I think the optimum slotting for this is 3x Recharge reductions so it can be used almost as soon as it's worn off, and 3x Defence dubuffs.
Afaik the def debuffs aren't autohit, they're applied once the target is hit, thus making later hits and debuffs more likely
Icelock - Ice/Storm Controller
Command Bot 1 - Bot/Traps MM
1 vote each way 
Anyone know if it's been actually tested? anyone wish to try testing it ? [Union]
I was thinking. If the debuffs are applied as often as the dammage, then youd have to floor the hit chance to 5% to get any real hope of not being debuffed.
Currently I have 2 rechrg, 1 def debuff and 1 slow in my instance of this power.
It seems to work fine even against purp mobs.
It's one of my favorite stormy powers. But I was wondering about slotting. I started by adding accuracy enhances , mainly because watching mobs fall over a lot was funny. But it occured to me that Deffence debuff might simply be better. As I'm assuming that the debuffs are actually auto hit, does anyone know?