Make CotC's shield bonus per enemy champion have diminishing returns.
CotC's current shield value at level 18 is 54 (+5% Max HP) per nearby enemy champion. This caps at 270 (+25% Max HP) near five enemies.
If you make the bonus shield decrease in value by 20% per additional enemy champion, though...
1 Enemy: 54 (+5% Max HP) 2 Enemies: 108 (+10% Max HP) / 97.2 (+9% Max HP) 3 Enemies: 162 (+15% Max HP) / 129.6 (+12% Max HP) 4 Enemies: 216 (+20% Max HP) / 151.2 (+14% Max HP) 5 Enemies: 270 (+25% Max HP) / 162 (+15% Max HP)
Not saying that'd solve all of the mastery's problems, but it seems like a start. 15% Max HP is still a pretty beefy shield for something you get passively.
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On an unrelated note, would making CotC not activate on knock-backs solve the issue of Vi and Hecarim being stupid with it, without really weakening it too much on the champions who are supposed to be using it? At the very least it keeps Vi from getting a 15% Max HP shield when she lands a Q.
(Does the game code differentiate between knock-ups and knock-backs?)