Say a player plays a Slip behind and then the next turn a Double strike... that's like playing a double strike alone, right? no effect from slip behind.
Also, any card that was set or waiting to do something, is discarded if the monster dies... A focus strenght, for example, doesn't stay there for the next fight, correct?
From the creator:
If you play SLIP BEHIND on turn one and a DOUBLE STRIKE on turn two, SLIP BEHIND will ride with the first of the two cards replacing Double Strike.
Note: If you play DOUBLE STRIKE and replace it with SLIP BEHIND and ATTACK 50, it will not immediately ride with the 50. You will strike for 50 and then double the card you play next turn.
Yes, set cards that are not played when an encounter ends are discarded. Potion effects end as well.