The hero you are fighting is Lord Shazar.
The Lord Shazar in your picture is level 55.
This means you are fighting him in Hard Mode Campaign.
Lord Shazar in Hard Mode Campaign has about 14k HP.
If you look at your Image 2, Lord Shazar has 14 Boxes.
Each Box represents 1k Health.
This means when you attacked Lord Shazar with your Outrider Champion.
You did 7k Damage.
This also means when you attacked with Outrider you used his A2 move.
Outrider A2 states "Attack 1 enemy. Decrease enemy MAX HP by 15% of damage dealt."
15% of 7k Damage is around 1k Damage.
This is why Lord Shazar only has 1 Box removed due to Destroy Mechanic.
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In conclusion, I think you are a lair.
I think you lied about doing 10k Damage.
Its clearly obvious you did only 7k Damage.
I think you lied about Destroy gear not working.
Its clearly obvious you didn't have Destroy gear on your Outrider.
You did the fight with Outrider with no Destroy gear set bonus.
You only used Outrider A2 move which does Destroy.
Than you went to take a Screen Shot of Outrider and saw your mistake.
You saw your Outrider didn't have a Destroy gear set bonus!
So you equpped him with the a brand new Destroy Weapon to get the Destroy Set Bonus.
Take a look at the rest of your gear - All +12 or +16.
Why is the weapon only +8?
Because you recently added it to your champion before taking the screenshot.