It's pretty simple.

Lets say you are an azure fang and you run Tobirama (+30% atk), Iruka (+30% atk/+30% nin), Mei lineup (+40% atk/+40% nin) (horrible one, but lets go with this one...) and you also use Water Enhancement (+40% atk/+40% nin), Skewering and Reserve Seal (+8% atk per hit suffered).

If your Azure Fang owns 10000 ninjutsu and 10000 taijutsu (i won't say 'attack' if not we could fall in misunderstandings), when everything is up you are considered like if you had (+140% atk, +110% nin) so you are like if you owned effectively 24000 taijutsu and 21000 ninjutsu.

Lets say in round 1 you get hit 4 times, then you get +8% atk per hit endured thanks to reserve seal passive, but +8% of what? Of that 10000 basic taijutsu you own, so you get a +3200 taijutsu and in round 2 you are like if you had 27200 taijutsu and 21000 ninjutsu.

Lets say in round 2 you use Skewering and that you are able to land it without enduring more hits.

Skewering is half tai/half nin, but how it scales with tai and nin? The formula is something like: damage = 0.5*(0.5*TAI + 0.5*NIN)*#RAN(0.75,1.25). So, the damage you land in that case will be (if the numbers are those ones) within the range of (9038, 15062).

Beware that those are numbers fully guessed but if are not the real ones are not far from them and would be the outcomes you would get against the extreme rich field cat (that owns 0 defense, 0 resistance, 0 elemental weaknesses and 0 damage reduction), not against a player.

What happens at the start of round 4?

You lose the bonus of the barrier, but iruka's, tobirama's and water enhancement are stil up for round 4 and 5, and lets say you were hit another 7 times.

So what's your bonus now?

+100% atk, +70% nin from the buffs and + 88% atk from reserve seal boost.

So you are considered like if you had: 28800 taijutsu and 17000 ninjutsu. So what will be the damage your skewering will land (given for good the numbers i said)?

11450 in average and you'll get numbers within the range of (8588, 14312).

What happens at the start of round 6?

You lose all the bonuses except for the stacked buff from reserve seal. Lets say you were hit another 8 times.

So right now you own only a +152% atk.

So at that point you are considered like if you had: 25200 taijutsu and 10000 ninjutsu.

what will be the damage of your skewering?

8800 in average and values in a range of (6600, 11000).

All buffs are additive, operated on your base stat. Having a two +40% buffs and a +30% buff give you a +110% buff in total. If one +40% buff is removed, you are left with +70%.

The only exception is wb asuma buff. This is a 1.3x damage multiplier, applied after all other damage calculations are done.