Pokemon GO's Buddy System has been a helpful tool for trainers to train their favorite Pokemon, and there are many ways to improve the bond between trainer and Pokemon.
One of the main sources of affection between a buddy and its trainer is for the Pokemon to battle while remaining a buddy. Players can earn quite a bit of affection from their Pokemon simply by using it in Pokemon GO battles.
Most battles serve the purpose of meeting the buddy system's requirement. However, there are some exceptions to be aware of as well.
Pokemon GO: Which battles count towards the buddy requirement?
When setting up your buddy to battle in Pokemon GO, trainers may want to fill its hunger bar first, which will place the Pokemon on the game map. Some trainers have maintained that they don't earn the necessary hearts if the Buddy Pokemon isn't traveling on the map with the trainer.
Once that's been taken care of, trainers will want to pick a Pokemon GO battle for their buddy to participate in. As it stands, the battles that will earn most affection from the buddy are gym battles, raids, trainer battles, training against the team leaders, and taking on Team GO Rocket members.
It's worth mentioning that the Pokemon itself won't need to fight in the battle, it just needs to be part of the battle party that goes into the fight.
The easiest route is training against the leaders of the various Pokemon GO teams. This can be accomplished by opening the battle screen from their menu and scrolling to the bottom of the battle screen. There, trainers can pick either Spark, Blanche, or Candela. Spark uses Electric-type Pokemon, Blanche uses Ice-types, and Candela focuses on Fire-types.
This allows trainers to focus on exploiting type weaknesses, and things should be pretty straightforward from there.
Using a buddy Pokemon to attack gym defenders will dole out affection hearts for each Pokemon defeated. If trainers see a packed gym with some weaker Pokemon defending them, their buddy Pokemon can earn multiple hearts by defeating opponents in the gym. This may be tougher than simply running training battles, but it serves as a great alternative to throwing down in Battle League.