First, let me start out with this, I'm not here to complain about how Shrike is overpowered.
But what I do believe, is that Sentinel should be a counter to Shrike. There are already many counters to shrike, such as: Mantle, stun, or shield. In fact, if you're prepared, you might not allow them an opportune moment to cast in the first place. But why not have another one? Why not promote the use of different shadow spells such as Sentinel? Sentinel and Seraph are probably not going to be called overpowered, unlike the controversial Shrike.
Sentinel takes -10% Damage, +80 Block, and Intercept. Sentinel is obviously a good spell when tanking for the team, or when spamming shields, or plague. But what about when you're facing a Shrike? Sentinel is a strong, "Football Turtle", as I like to call it. It should be able to withstand some damage from a Shrike. When someone casts a Shrike in 1v1, I feel like playing a Sentinel would help me, but it doesn't.
What I suggest, is perhaps making Sentinel take -15% to 20% less incoming damage. Some of you must be taken aback by this suggestion, but I recommend this for good reason. In PVE, you'll feel like more of a tank, without actually being insanely overpowered. Who likes tanking? I do, but not many people do. This will promote tanking and team play. It will also serve as somewhat of a counter to Shrike.
-10% Incoming Damage is equal to 10 non pierce able resist. When you get hit by a Shrike, you have 10 Resist, absolutely nothing, you'd better off not even using it, due to backlash and losing flexibility in card play. 20 Resist is what most Glass Cannons have, and is still near nothing, but it is worth it. If I get hit by a 2000 Damage Artorius, I'd take 1600. This is at least something I can do as an alternative to using Fortify, shielding, or mantling all together.
I think Sentinel needs a buff, and of course this is going to get tons of criticism. A lot of people might say Shadow Magic is fine as it is. I believe everything in Wizard101 is overpowered, and that makes everything fair. But a buff to Sentinel wouldn't throw shadow magic off balance, and would promote tanking and team play.