After taking a closer look at Creeps Armour and Mitigations in order to Min/Max corruptions I noticed something was off.
After closer inspection I realised that literally nothing about Armour, Mitigations and their respective Calculations are working as described in the stat panel.
This may be due to Creeps being based off of NPC's. But with current Freep debuffs being quite potent and Creep base stats fairly low, this has massive effects on incoming damage.
This is going to be a very long post. There's a ''Conclusion or The TL;DR'' if you scroll down a bit.
Lets take a look at the base stats first.
Base Stats
Here are the current base stats of a Reaver without any corruptions, potions or other outside buffs.
The only buff present is the passive 20% to all mitigations from Relentless, as long as you are on full morale. Which doesn't influence Rating, just %'s.

This will be our reference sheet for all following tests and calculations.
Upper Hand
An easy way for our Reaver to increase Armour Rating is the buff from Upper Hand.
Lets apply Upper Hand to our Reaver and see what it does to our Armour and Mitigation values.

Lets compare this to our reference sheet. We will find that:
- Armour increased by 7000
- Physical Mitigation increased by 7000
- Ancient Dwarf, Beleriand and Westernesse Mitigations increased by 7000
- Tactical Mitigation increased by 7000
- Fire, Light, Lightning, Frost, Acid and Shadow Mitigations increased by 7000
From this we can conclude one of two things:
- The Armour calculation is not working properly and is adding more than it should.
- Creeps only have Common Mitigations and don't have any Non-Common ones.
If option 2 is the case, then that means that all specific mitigations should have the same Rating as Armour, or 14.000. Unless there's still factors applying a negative amount to them in some way.
But what happens when instead of an Armour Buff, you get a Debuff?
Puncture Target
In order to determine what happens when you get debuffed we had our Reaver get hit by a Puncture Target from a Blackarrow.

As you can see from the results, every single mitigation dropped by exactly 6416. This again confirms that any increase or reduction to Armour gets added or subtracted 1:1 to/from mitigation values.
Lets take this a step further.
Sundering Blow
Lets see what would happen if our Reaver gets hit with a very potent debuff. For this we brought in another Reaver, to apply Sundering Blow.

As one can see, mitigations still dropped by exactly 36.642. What's more interesting is that even though we only have 14.000 armour, the mitigation decrease didn't stop at 14.000.
This single armour debuff leaves us with 0% to all mitigations.
Damage Taken
On to the next question. Is it currently possible to go below 0 mitigations, and what effect would this potentially have?
In order to test this we need a 2nd Reaver. Why a Creep? Because Creep damage is static and thus doesn't fluctuate between values, making it easier to get results.
I have tested a couple of skills to see what kind of effect it has. Ravage and Sudden Strikes. Low cooldown, low damage. Blade Toss, high hits. And Impale, to see how a Mitigation bypass deals with this.
How did I test it? Have one Reaver slot Burning Blades. Burning Blades turns Orc-Craft damage into Fire damage, which is needed because Creeps don't have a mitigation tooltip for Orc-Craft.
Then have Reaver-1 attack Reaver-2 with a skill a couple times. Hit Sundering Blow and hit the skill a couple times again.
Now repeat the entire process with 3 Tactical Mitigation Corruptions slotted (22.500 additional Fire Mitigation). This gives enough mitigations for a clear change in damage but not enough to exceed the debuff from Sundering Blow. In both situations we're left with 0% mitigations after applying Sundering Blow.
Here are the results.
Sudden Strikes
No Mitigations Slotted
This is with a base Fire Mitigation of 10.000 or 36,1%
Lugbur scored a hit with Improved Sudden Strikes on Urundus for 1,055 Fire damage to Morale.
Lugbur scored a hit with Improved Sudden Strikes on Urundus for 789 Fire damage to Morale.
Then after using Sundering Blow, which leaves us at 0% Fire Mitigation.
Lugbur scored a hit with Improved Sudden Strikes on Urundus for 1,321 Fire damage to Morale.
Lugbur scored a hit with Improved Sudden Strikes on Urundus for 988 Fire damage to Morale.
3 Tacticals Slotted
This gives us a rating of 32.500 or 64,3%
Lugbur scored a hit with Improved Sudden Strikes on Urundus for 589 Fire damage to Morale.
Lugbur scored a hit with Improved Sudden Strikes on Urundus for 440 Fire damage to Morale.
Then after using Sundering Blow, which again leaves us at 0% Fire Mitigation.
Lugbur scored a hit with Improved Sudden Strikes on Urundus for 1,238 Fire damage to Morale.
Lugbur scored a hit with Improved Sudden Strikes on Urundus for 926 Fire damage to Morale.
As you can see, even though we have a tooltip value of 0% in both cases, there is a difference in damage.
Lets try another one.
Ravage
Again, without and with Mitigations, and Sundering Blow applied to both.
Without Mitigations:
Lugbur scored a hit with Ravage on Urundus for 1,211 Fire damage to Morale.
Lugbur scored a hit with Ravage on Urundus for 811 Fire damage to Morale.
Lugbur tried to use Ravage on Urundus but he evaded the attempt.
With Mitigations:
Lugbur scored a hit with Ravage on Urundus for 1,136 Fire damage to Morale.
Lugbur scored a hit with Ravage on Urundus for 760 Fire damage to Morale.
Lugbur scored a hit with Ravage on Urundus for 760 Fire damage to Morale.
Next up. Blade Toss. Same situation.
Blade Toss
Without Mitigations:
Lugbur scored a hit with Blade Toss on Urundus for 4,023 Fire damage to Morale.
With Mitigations:
Lugbur scored a hit with Blade Toss on Urundus for 3,214 Fire damage to Morale.
Last one. Impale. It gets interesting here.
Impale
No Mitigations
Lugbur scored a hit with Impale on Urundus for 4,029 Fire damage to Morale.
With Sundering Blow:
Lugbur scored a hit with Impale on Urundus for 4,290 Fire damage to Morale.
3 Tacticals Slotted
Lugbur scored a hit with Impale on Urundus for 2,817 Fire damage to Morale.
With Sundering Blow:
Lugbur scored a hit with Impale on Urundus for 4,290 Fire damage to Morale.
Calculations
Now here comes the fun part. Math! Lets calculate the theoretical amount of damage we're supposed to receive being at 0% using the Pre-Sundering Blow damage values and their respective amount of mitigations.
For this I'm using: ReceivedHit / (100% - CurrentMitigationValue). For Impale we're subtracting another 30% from CurentMitigationValue to account for the Mitigation Penetration the skill has.

As you can see the calculated values don't exactly match up with the damage received, with Impale being the only exception.
Now what about passive mitigation %'s (from Relentless and using 3 corruptions)?
TheoreticalHit * (100% - MitigationIncrease). For Impale we're adding another 30% for the penetration. This will exceed the 0% cap though, so the number will turn out higher than it should.

Now we see the numbers match up pretty well with the in-game obtained values. With a few points difference, most likely accountable to rounding off.
So it's safe to say that mitigations indeed cannot go below 0%.
What also can be learned from this is that the tooltip is currently wrong.
Even if you are debuffed to 0 rating, % changes will still apply, though not visible on the character sheet.
Conclusion or The TL;DR
Now what can we conclude from all of this?
- Increases and Reductions to Armour directly apply to all Mitigations 1:1.
- Current Base Mitigations are lower than they should be, in comparison to what Armour should add to them.
- Reductions to Armour still apply to Mitigations 1:1 even if the debuff exceeds the total Armour value.
- It is not possible to go below 0% Mitigations.
- The tooltip for Mitigations is currently not displaying % increases correctly if you are debuffed to 0%. Passive % Mitigation increases (such as Relentless on Reaver and +5% from using 3 Mitigation Corruptions) still apply to the calculation even though not shown.
How I think it should work
Armour and Mitigation calculations currently work way different on Creeps compared to Freeps.
In my opinion Creeps should have mitigations calculated in a similar way as Freeps do.
Armour
Creeps should have a base amount of armour, depending on the class played. This gets increased by Consumables, Buffs and Traits slotted.
These numbers should be vastly increased compared to what they currently are to accommodate for the following changes.
Physical Mitigation or Common Damage
Creeps should receive a base amount of Physical Mitigation depending on the class. (This is currently 25.000 as Reaver but barely does anything since it only affects common damage)
100% of the current Armour value should be added on top of this.
Ancient Dwarf, Beleriand and Westernesse Mitigations
All of the Base Physical Mitigation (current 25k) should be added to these mitigations.
Then, 20% of the current Armour value on top of that.
Tactical Mitigation
Creeps should receive a base amount of Tactical Mitigation. (This is currently 0)
Then, 100% of Armour Rating added to it.
Fire, Light, Lightning, Frost, Acid and Shadow Mitigations
Creeps should get some class specific passive ratings in a few of these mitigations.
All of the Base Tactical Mitigation should be added on top of these.
Then add 20% of the Armour Value.
What would be the result then?
Armour would gain a large increase, but it's effectiveness towards the non-common mitigations reduced. Debuffs will no longer exceed the total Armour value.
Due to Armour being less potent, current base mitigations would need an increase and base Physical Mitigation an adjustment downwards.
With Armour being less potent, Armour Debuffs will be as well.
Creeps will have higher defences and be able to better cope with the ever increasing Freep damage output and debuff potency.
It will be much easier to scale Creeps accordingly by simply adjusting the Base amounts of Armour and Mitigations when needed. (E.g. after a new level cap)