Knock out 3 birds while they are attacking your Palladin?

Can you give more context? It’s hard to figure out what part of the game you’re talking about.

ETA: I’m guessing you’re referring to an arena objective, but the only one I’ve ever seen that sounds close to that is “knock out three birds while they’re attacking your BANNER (not your Paladin).”
If that’s what it actually is, then your strategy of using Mage’s Shock Shield spell should work, but Rogues’ Cupcake Trap is better (both at the same time even moreso!). You can also use a Spiky Spike (aka “Thorns”) emblem on your banner. Avenger and Princess can also force your enemies to attack your banner instead of (or in addition to) Rogues.
Last tip is to make sure you can adequately heal your banner too, because you have to win the battle to get credit for the achievement, of course!
My suggestion for this achievement is to use Rogues, Witch, and pretty much any third bird you want (I guess pick depending on the enemy team’s strengths/weaknesses).