Saw the W notes on PBE. No. Just No.
I understand you think she has too much agency after your buffs. This is not the right fix. if anything, W needs a buff in damage, not reduction, to maintain mage status, compared to the rest of the meta.
Here's how you bring her down:
_**1) Revert the Season 7 end buff to her Binding (Q) Cooldown by 2 seconds. **_
If it's less spammable, she's less reliable, and less frequent at getting someone stuck on her pool. It also reduces the frequency of her W cooldown reduction.
_**2) Reduce Black Shield's Magic damage absorbed per tier by 10 to 60 (60/120/180/240/300)**_
BUT: Raise W to 18% AP Scaling.