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.

It's hard to take your post seriously, as you are either an unfunny troll or delusional.

The 10.000 damage was simply an example because it's an easy case of taking a 40% cut (4.000) without using a calculator. 7.000 * 0.4 = 2.800 damage reduction, not the case in the image either. I have repeatedly tested Outrider and each time it never reduced the 40%, no matter what skill I used. You can test it for yourself and will probably have the same results. I uploaded the images to make my point easier understandable to readers. 

Why would I roll my weapon further than 8 for the time being? My goal is to first increase health and defense so I can surive long enough to reduce enough of Scarab King's maximum health. Only one piece of my gear is +16, and it's not even from the Destroy set... what's your point? The chest and boots are obviously important pieces, and the helmet is on +12 for the extra health.

I don't understand the purpose of your post. All you do is distract from my original point, which is that the set may be bugged in campaign and potentially other places. It would be nice if someone who is actually credible tested this, so we can resolve this. Btw, I'm using a Samsung Galaxy S7 edge.

I remember, that I've heared in a youtube-video the reduce-max-hp-effect is capped at 8% of the total enemy hp per Attack. That would 

1. explain the effect ToFu96 recognized without the stupid accusation Player J made

2. clarify that the set is totally useless.

/edit: 

 When attacking with a Destroy Set, the target’s MAX HP cannot be decreased by more than 8% with each attack.  

From the basic-guide-artifacts-overview.

Other than now in Doom Tower, of course. 

Thanks guys. I guess that is information you'd expect to be in the set's description. Since you say "8%", I quickly analysed my health bar image.

• The total health bar (excluding its borders) is roughly 153 pixels
• The max HP reduction is about 12 pixels

Now we can do a simple calculation

• 12 / 153 = Roughly 8%

Your statement corresponds with my scenario. Some further, irrelevant calcuations

• The damage is about 77 pixels 
• 77 / 153 = I dealt about 50% damage 
• 77 * 0.4 = I expected 31 pixels of damage reduction, 31 / 153 = 20% of total enemy health. 14.000 * 0.2 = 2800 max HP reduction, as I originally expected.

And yes, the set is terrible. I wonder why the max reduction for each attack is capped at 8% though.