The Battle Frontier is a feature(usually used post-game) for players to battle other (usually more powerful) trainers. There are many different parts of the Battle Frontier, including the Battle Tower and the Battle Factory
The events and scripts for the battle tower are found in the example map "Battle Frontier". The information for the trainers and the Pokemon that they use are in the PBS files bttrainers and btpokemon respectively.
The battle factory is a resource where players use borrowed Pokemon to fight. Unfortunately, the scripts for the Battle Factory do not work in the current version of Essentials(v16.2)