Check Looking Direction Of An Entity

Hi! I am creating a mod that adds a lot of entities, and I want to create one that has a huge shield in front of it.

I created my model and added the mob. It parries projectiles when they arrive and looks in the direction of the source.

Now, I want it to cancel sword damage when it is looking at the player (so it can only be hit from the back), but I can't find the procedure block to do it.

Do you know a solution to my problem, please?

Thanks! :)