Don't be transferring to a different map. That's a no-no. Switches and variables are two of the most fundamental concepts in eventing. They are worth taking the time to learn.
The first image below is the enemy event. Notice at [1] that I turn the A self-switch ON. At [2] you then create a new event page (notice at [3] that we are currently ON page 1).
The second image is of the second page of the enemy event (notice at [2]). Now, at [1], see how I check as a condition that self-switch A be on.
Pages work basically that the highest number page that meets conditions will run. As long as Self-switch A is OFF, that will be Page 1 since Page 2 doesn't meet conditions. As soon as self-switch A is turned ON, however, the event will go to page 2 since the condition is now met.
If you are performing the battle in a separate event, then you just need to use a regular switch instead of a self-switch.