BUFFS, DEBUFF AND DIMINISHING RETURNS GUIDE

A) How multiple of the same buffs/debuffs interacts? What is "diminishing returns"?
Now, lets start with the basic. There are various buffs and debuff that can increase or lower your stats. However, if a same type of buffs or debuff were applied to the same target, instead of adding everything up (A+B+C+D ect), it will use this formula: [A + (A*%B=X) + (X*%C=Y) + (Y*%D=Z) ...ect] where:

A= The highest buff/debuff percentage
B= The second highest buff/debuff percentage
C= The third highest buff/debuff percentage..ect

Example: Krervo cast "Active Liquid" that gives +20% Int and Lele cast "Leave it to me!" that gives +30% Int. So Lele has 2 different int buffs. But instead of 30% + 20% = 50% increase, it will take the highest value as base, then apply the formula: 30 + (30*20%=6) = 36%
Example 2: Now aside from Krevo and Lele's buff, you also swap in Ciel with his lvl 2 valor chant. Now Lele has 3 int buffs: 30%, 20% and 20%. You applies the same formula to get: 30 + (30*0.2=6) + (6*0.2=1.2) = 37.2%

As you can see here, due to diminishing return, the second buff only adds 6%, while the third buff only adds a measly 1.2%, completely not worth the trouble of wasting a turn to apply the those extra buff.
Based on this calculation, we can also concludes that the higher the buff/debuff percentage, the more valuable it is. Let say we have two types of scenario. One with 40%,30%,20% buffs and another with 30%,30%,30%. Both add up to 90% total. But if we applies the calculation, it will be :

Case 1 = 40 + (40*0.3=12) + (12*0.2=2.4) = 54.4%
Case 2 = 30 + (30*0.3=9) + (9*0.3=2.7) = 41.7%

A) 12.3% difference! So, the higher % of your strongest buff, the better. Mariel is so broken!
In future content, there will also be buffs that can stacks. Like Aldo's 20% slash resist debuff that can stack 3 times. But after applying the first stack, all future stack will be half, so instead of 20%+20%+20% = 60% for 3 stacks, it will be 20% + 10% + 10% = 40%.

B) The type of damage in this game, how their debuffs/buffs interacts
Now we know about how buff and debuff's diminishing return works. Lets get into the more advance stuffs.
The game has two damage types, physical and magical damage. And it gets further divided into various element and weapon types. Physical damage type has pierce, blunt and slash. Magic damage type has fire, water, earth, wind. All type resist means all magic damage types are included, same as physical resist.
Then there are buffs/debuffs that increases or lower your physical or type resist. The same formula also applies to these type of buffs/debuffs. However, the game will calculate the physical portion of the buffs/debuffs and magical portion of buffs/debuffs separately, then add both together. What do I mean by that? Well, there is something called elemental + weapon attack, Like Otoha's Earth slash attack. So if I were to use Otoha's skill and attack a target with slash resist debuff, earth resist debuff, or earth + slash resist debuff, how do I calculate how much % damage increase I get? Lets look at some example.
Example 1: only slash resist debuff. So the enemy has 30% and 20% slash resist debuff. The calculation is simple: 30+(30*0.2) = 36% damage increase
Example 2: only earth resist debuff. So the enemy has 30% and 20% earth resist debuff. It is the same deal: 30+(30*0.2) = 36% damage increase
Example 3: both slash resist and earth resist debuff. So the enemy has 30% earth resist debuff and 20% slash resist debuff. It will be 30+20 = 50% damage increase
So a mixed of both elemental resist buff/debuff and physical resist buff/debuff will net you greater result if you use elemental physical skills.

C) Damage mitigation: All type resist up, physical resist up, power debuff, int debuff.
Now damage mitigation follows the same principle:
Example 1: 50% All resist up + 30% All resist up.50 + (50*0.3)= 65% reduced magical damage
Example 2: 50% All resist up + 15% physical resist up-If magic attack: 50%-If fire physical attack: 50 +15 = 65%
Example 3: 50% All resist up + 50% fire resist up + 30% physical resist up + 15% slash resist up-If fire slash attack: [50 + (50*0.5=25)] + [30 + (30*0.15=4.5)] = 75 + 34.5 = 109.5%-If fire magic attack: ~75%-If wind slash attack: ~84.5%-If wind magic attack: 50%-If slash attack: ~34.5%-If blunt attack: 30%
As you can see here, a mixed of both physical and all type resist up can have drastic effect should you receive elemental physical damage. If your resist % reach 100%, the attack will deal 0 damage.

How about PWR and INT debuff?
This is based on info collected from Japanese blog, Chinese forum and my own data collected from fighting Azami. A 20% power or int debuff is roughly equivalent to about ~50% damage reduction (or 1% power/int debuff = 2.5% damage reduction). Making power and int debuff the most efficient way in reducing damage from the enemies. Power debuff are especially important, when we don't have anything that buff physical resist by 50% to all party.

Should I stack pwr and int debuff? Stack resist buff? Both?
2x pwr debuff* (25% + 20%), we will get about ~30% power down on enemy, which is equivalent to ~75% physical damage reduction.*25% pwr debuff from Anabel, 20% pwr debuff from Ciel
2x int debuff* (25% + 20%), we will get ~30% int down, which is ~75% magical damage reduction*25% int debuff from Azami VC, 20% int debuff from Anabel
2x type resist* (50% + 30%), we will get ~65% magical damage reduction.*All type resist buff from Mariel and Ciel
2x type resist* (30% + 30%), we will get ~39% magical damage reduction*All type resist buff from Ciel and Feinne
2x physical resist* (35% + 30%), we will get ~45.5% physical damage reduction.*Physical resist buff from Anabel VC + Bivette
2x physical resist* (30% + 30%), we will get ~39% physical damage reduction.*Physical resist buff from Aldo VC + Bivette
Mix pwr + physical resist*(25% + 30% ), we will get ~81.25% physical damage reduction.*Power debuff from Anabel, physical resist buff from Bivette
Mix pwr + physical resist* (20% + 30%), we will get ~65% physical damage reduction.*Power debuff from Ciel, physical resist buff from Bivette
Mix int + all type resist (25% + 50%), we will get ~93.75% magical damage reduction.*Int debuff from Azami VC, all type resist buff from Mariel.
Mix int + all type resist (25% + 30%), we will get ~81.25% magical damage reduction.*Int debuff from Azami VC, all type resist buff from Ciel.
All the buffs and debuff are taken from the strongest value whenever possible. Mariel's Aurora Force is an outlier, so I do extra calculation if you don't have access to Mariel's Aurora Force.

So the conclusion is:
Power/int debuff + physical/type resist buff > stacking power/int debuff > a single power/int debuff = Mariel's shields > stacking physical/type resist buff without Mariel's shield > a single physical/type resist buff without Mariel's shield > no buffs/debuffs

Practical test:
In Azami's fight, her triple wind slash can deal upwards to 6k damage against a lvl 60 equipped character. One-shotting everyone. Even with Mariel's shield, it still do 3k damage, one-shotting all 4* characters. But if you combine Bivette + Mariel's shield, you will have a 80% damage reduction, making her wind slash to deal around 1200 damage. And if you combine Soira's 70% physical damage reduction + Mariel 50% all type resist up, it will be 120% damage reduction, so her 3x wind slash deals 0 damage to Soira (but still 3k damage to everyone else).
Also in Azami's fight, Azami auto attacks/wind slash hits for about 2.2k damage per hit, with just a single power down from Ciel, it cut the damage in half, resulting in 1.1k damage average. And if you apply Bivette's physical resist up, it will further cut the damage down to ~770 damage. Combine it with Ciel all resist up buff, you reduce it even further to ~539 damage per hit. Making it very survivable even if she uses her 3x wind slash. However, her normal non elemental physical attack are still at around 770 damage since it is not affected by all resist buff.

Buffs and debuffs have diminishing returns, casting multiple same buffs/debuffs is inefficient. The only exception is stacking multiple defensive buffs/debuff since every damage you can mitigate means you will more likely to survive from the enemies' attacks. It also makes your healer happy.
The more powerful the buff/debuff, the better. Stacking a 30% + 10% buff is better than 20% + 20% buff, due to the diminishing return formula takes the highest % value to calculate their final value. Part of the reason why Mariel is so broken.
Elemental physical attack benefits the most from both physical resist debuff + type resist debuff.

Combining all type resist buff + physical resist buff can greatly mitigate enemy's elemental physical attack (Look at practical uses section.)
Power/int debuff are the most efficient in reducing enemy's damage. Aside from Mariel's Aurora Force.

For damage mitigation, Power/int debuff + physical/type resist buff > stacking power/int debuff > a single power/int debuff = Mariel's shields > stacking physical/type resist buff without Mariel's shield > a single physical/type resist buff without Mariel's shield > no buffs/debuffs
For buff/debuff that stack X times. First stack applies the full %, all future stack will be halved.