For the second mission, If you face somebody who is obviously doing the 3rd mission, no pill on the first round. More then likely they'll go all in and then you can sit back and get easy pills.
On the 3rd mission you don't have to use piranhas, any cards with -x opp pills will do. Eklore also counts but you have to be outside the warehouse to use her.
4th mission you can just play regularly but I advise playing in lost warehouse with a undeveloped piranhas card, sliman works well for that role with 2/6 and 3/7 on his 3rd and 4th level, respectively.
For the third mission, I used Junkz. Tremorh, Gil, Neil and Eklore are easy to use and can all spell hell for an unprepared Piranhas deck. Two pillz on anyone with Eklore out will either lead to an easy win or massive overpilling from your opponent.