Heavy Stun and Endurance Bar? (raise shield)

How do these work?

I know that stun threshold just raises the cap of when you will be stunned from taking a hit. But does this also scale with the endurance bar?

What i'm particularly interested in, is how can I block(raise shield with resonating shield), without the endurance bar getting filled so fast and prevent being heavy stunned?

I'm doing a resonating shield build but im still early game and I noticed that if there are many mobs around and i don't do damage fast enough, they heavy stun me. Which is making me kinda anxious for mapping.