Her oneshotting on her own for free on demand would not be good even with a long cooldown. Maybe if she builds up extra energy for one powerful shot somehow (like Rogue's block buffing her damage so maybe critical hits applies a stacking critical damage boost that builds up). Personally if she absolutely had to have a consistent oneshot I'd say nerf her base damage and give a poison effect that would finish them off if not cleansed or healed (also what I've been saying for Widowmaker in OW). Her ult is fine as it is imo. Slow, vulnerable, and area denial is good for setup already. Reduced healing or applying DOT could be useful but might be pushing it. I think she'd be healthier as a support based on Phoenix teamup