I've just tried killing her and saw something not normal, i was tanking (Druid) the whelps and when p2 started (with the bubble)
i got like 300k attack power from Vengeance (insane dps i reached 100k).
now this is a bug since you don't get that much damage to stack it this high.
The group i was with were very bad, but you could actually do some nasty damage on Sinestra if dps is high enough.
basically what i was doing was taking whelps and just doing 1 thrash while it crit for 200k. then help on sinestra shield and sinestra itself on the tries the dps were high enough.
so basically if your dps in group is high enough you could actually help dps sinestra in p3 (and see big crits).