Well, let's see...you could add a Script to the Battle Event page and write something like this:
CODE
$game_troop.members[index].hide
to hide one of the enemies (index is to be replaced with the enemy's index, 0 being the first one);
CODE
$game_troop.members[index].appear
to make one hidden enemy appear halfway (index is to be replaced with the enemy's index, 0 being the first one);
I hope this can help,
Jens