the havadca dabra is a very inaccurate move. The time it takes to do that move is two seconds. the curse is a forbidian move it no one has the luxery or the odasity of perfecting it. only few can do it which make worst. it likely will not effect those with cartoon physics. They would play along by being hams to act like they are dying. so it wont effect mxyzsptlk or the mask.
Thanos would welcoming to himself to be vulnerable. if anyone remember the cartoon when daffy acts like William Tell while Elmer shoots the apple. it will go like that. if it hits him he is a titan so uncertain. I think make him fall asleep or a coma. same thing with Darksied
It would be more believable that it would kill him than how DC done to kill him. this would open to phantom superman. a super ghost would take into effect. It would be more interesting of him being revive,