Boss Battle Specialist is an achievement in Plants vs. Zombies: Garden Warfare 2 that was introduced in the Trials of Gnomus DLC. To earn it, the player has to enter the Mystery Portal inside the Town Hall and defeat a boss in Boss Hunt on Hard difficulty.
It's quite difficult to fight a boss alone, so be sure to queue into an active game, and have a full team. You could even play with friends, so that you have much better communication in the game. The most important tip in all of this, is to not fight a boss from the Mystery Portal alone. It's highly recommend to achieve this with friends, or at least other players.
- It is comparable to the Garden Hardcore achievement in Plants vs. Zombies: Garden Warfare, as both need the player to complete a mode on Hard difficulty.