I hate it when people ask this
just carefully work out the highest attack your opp can get, then work on from their.
e.g. i have Kolos (7 power) and opp has DJ Korr Cr (8 power)
he has 5 pillz (8x5=40 (+8 more for Junkz bonus) =48)) 48 is the highest poss attack
i have 10 pillz (7x7=49 (beat him by 1 attack, and saving 3 pillz))
But, alot of pil playing is just luck, mainly in earlier rounds.
Some trolls would say that keep in mind that you want to win the fight.
So you should first think of how you can achive that: which are your strong cards, and which are the opponent harmful cards.
Then you have to hit with your strong ones and at the meantime you have to avoid being hit too many times.
Next lesson is bluffing.
Pick which rounds you want to win and use the minimum amount of pills to achieve the goal.
This question, as you will grow to learn, has no straight forward answer, but never put more then 6 pillz on a card that wont win you the game that move, generally, i see player using 1-5 pillz opening move, then there are so many outcomes from that first round, your just gonna have to work it out yourself from there, another suggestion, fury is usually not worth it, so dont use it unless absolutley nessacary
There is no 'magic formula' to play your pillz every game. It all depends on the in game situation and what you think the opponent is going to do. Try to counter what you think he might do by either spending enough pillz to beat him or bluffing to make him waste pillz. Once you play for a while, you will get a feel of what to do and what most players do, or in one word, experience. Good luck!