Now the "pick by comparison" that CC has is like the "system compare" with these differences:
1. Corresponding picked instances are used for expression values.
ex:
The first picked enemy is compared with the first picked player,
the second with the second,
the third with the third... and so forth.
If the amount of picked objects differ in count for example: 2 players and 4 enemies it will loop back to the first.
ex:
The first picked enemy is compared with the first picked player,
the second with the second,
the third with the first,
and the fourth with the second.
2. The enemy objects are picked.
I'd also like to point out these apply to picking in general with most object conditions.
Now for the workarounds until "pick by comparison" is added.
Since I assume there will be one player and many enemies Yann's second solution will work fine. I've also used private variables to do the same thing:
Every Tick-> enemy: set dist to abs(player.x-enemy.x)
enemy: dist < distance -> do stuff