I want to give some more information about physical attack vs. defence of the boss. First of all the formula postet by Undrentide(originally by Andreasels) is correct.
However, most of the people are not interested in calculations so I made a graph of the boss deff in relation to your own patt.
From the graph you see three different areas.
I. Patt below 20%:
If your patt is below 20% of the boss pdeff you deal a fixed amount of damage, depending on your weapon and damage attributes.
This is why tanks don't need to stat Patt at all and can still make damage.
II. Linear Range:
The next point where the formula changes is where your own patt equals the deff of the boss. In this range your damage increases linear with your own patt.
III. Patt > Pdeff:
Here your damage increase doesn't grow linear with your Patt. In fact if your Ratio Patt/Pdeff grows you'll get closer to the max value of 2* MaxDamage. But as you can see in the graph, your damage is still increasing significally while your patt is not way higher than the bosses pdeff.
To conclude, I want to relate these values to the actual content on Arcadia.
1. Goblin, Cenedril, old content like medium difficulty HoS, HoDL, ZurdS:
Your Patt should be way higher then the boss pdeff so it is almost irrelevant.
2. Dalanis Nightmare:
Here I found out, that the deff value of the boss must be very very high. I am always in range one, meaning that stating or buffing patt makes absolutly no difference. You can test it yourself and compare if buffing patt gives you any damage increase. If it does, write me
3. Tikal and future instances.
The exact deff values off Tikal are a secret but knowing the formula and your own patt values you can calculate the boss deff. I can reveal that it lies somewhere between 1kk and 1,5kk This means that you will either have to debuff the boss to decrease its deff or you have to buff yourself to 1kk patt to get to the third range. And even there, every increase in patt will still benefit your damage.