If you put her CD from 3 to 5, players who chose her would feel betrayed from this huge gap and will ask for compensation.
But I like the "one target only" idea. The AoE is just illogical for a 3 turns CD that kills anything. Or/And remove the damn poison.
Putting units to 1 HP is insane enough.
I think the feasible solutions could be:
a. Make it single targeted. This will still be broken as I assume this will bypass 100% immunity, heal x% of damage dealt(like Snow White's skill) but this means that a square ranger formation would still function, even with one of them down. By making it single targeted, this still doesn't negate it being a ridiculous spell but I feel like this is already manageable for opposing team.
b. Make the damage instances separate. What I mean is 'x% chance to decrease enemy HP by 33% for 3 turns'. This means the spell will still be powerful but it wouldn't feel like it's broken. In essence, that cuts off 99% of a hero's HP but you need some luck on your side more than before. Still, it's a high reward for a low CD spell.
I think making it 5 turn CD makes it kinda just an okay spell, not DL worthy imo. I don't even own Cass so I have no reason to defend her but by turn 5, the enemy could already counter your Cass in so many ways like adding CD, stunning, etc. On a side note, it should be dispelable as this skill doesn't have a counter once it is already cast. That, in itself, already screams OP.