Power is the strength of which your card makes its attacks. It's written there on the card, eg. Dorian has a power of 8 and a damage of 7. And attack is the total of your cards power multiplied by the number of pills plus the original power.
Example : Power = x, Pills =y and we'll use Dorian.
So x = 8 (Dorian's power) and lets say we use 4 pills, so y=4, thus, you have :
(8*4) 8 = 40, so the attack is 40.
Hope that is clear for you.
So to answer your question, All Stars has the bonus of -2 Opp Power, in Dorian's case, his power will be reduced to 6. Any card's power will only be reduced to a minimum of 1. So if a card's power is below 0, 1, 2 and 3, it'll always be at 1.