Just change the mechanic in general. Have it deal a considerably lower base damage, but have the base damage (not the scaling) ramp up if she's got a lower % health than the target, increasing based on how high the % health differential is.
Furthermore, remove the potential for a stun altogether. Instead, replace it with a slow when her % health is lower than the target's % health, with the strength of the slow also scaling based on the differential. Additionally, make this ability always apply a new form of cc (we'll call it 'Dizzy') that prevents the usage of dash/flash type abilities while it persists. This cc always gets applied, whether she's got more or less % health than the target.
So, if she's higher, it's just a dash/flash disable with low damage and no other slow or cc effect. If she's lower, it'll disable dash/flash type abilities and slow/damage based on how far behind she is (making it more and more potent the riskier it is for her to be in the situation).
(And this isn't the only ability I'd want this 'dizzy' effect placed on, I think it should be spread around on many melees, being used in particular either as a replacement for oppressive cc's like Irelia's stun or Talon's former silence or to help lower-mobility melees stay relevant in a game of mobility, for example adding it to Darius's W or Morde's Q)