CotC nerfs incoming

Having it scale on resistances would mean exponential scaling.

Let's say that your supposed Colossus scales like Maw's Shield, 1:1 with Armor and MR. Also, the CoC user in question has 100 Armor and, for the easiness of the argument, 100 shield strength, giving him an effective shield strength against physical damage (epHP) of 200. With the additional 100 Armor, his shields strength is bumped to 200, and his Armor to 200 as well. However, the epHP would become 600, tripling the previous amount.

With it scaling on HP, you have linear scaling. Same example, but shield scales 1:1 with HP. Initially, user has 100 HP and 100 Armor, and a shield strength of 100 and epHP of 200. Adding an additional 100 HP, thus doubling the user's HP value, gives a shield strength of 200 and an epHP of 400, doubling the previous amount.