The shield can provide cover from grumpy bomb allowing you to avoid the stun.

Unless the Bomb King is using Demolition, use your shield to block grumpy bomb.

Use legendary Demolition to counter all of his shields, even the ultimate, making it useless. This legendary might not be the best choice, but if opposite team has 2 or more tanks, including Barik, this legendary becomes even more viable.

Grumpy Bomb combined with Demolition legendary is a nightmare for Barik, especially if he's with another front liner. That's because he likes to set himself up in one place, and you are forcing him out every 10 seconds.