[Snake]

There are quite the few ways to have 'ninja' traps being put in the field without the enemy knowing. Such as small conditions, like.

"Whenever you're knocked down, knocked back, or walk X amount of tiles, a random trap is laid down on your spot. Whenever you score a critical hit behind the enemy's back. Whenever you knock the enemy down." Et cetera.

This is the alternative to having their damage up. The 'what if they don't cost momentum sometimes' option.