So, I love everything except for:
His "shield" is weird, and it's hard to wrap my head around it, so I'll do what I always do and use math.
Let's say that Taric has 1700 bonus health (near end-game build, including runes+masteries), and my teammate has 2000 health.
The shield would be worth 2180. Now taking into account the damage reduction, 50% of the damage is directed towards the shield, the other half towards the normal health of my teammate. With no resistances, they already have double the health.
With the average resistances of a late-game carry, let's say 75 armor and 30 MR, they will have between 7135 and 5434 health respectively.
They can get little over 250% to 170% more health for 5 whole seconds. It would only get worse with tanks, who actually build for resistances.
You could easily invalidate half the damage of any burst with this one spell.
If this is not the case, please refrain from calling it a shield and improve your description, that's very confusing.
His ultimate has too much.
It does what Braum's does, but infinitely better.