Complex sentence: Charmander evolved into Charmeleon during a battle against an army of Exeggutor, and his personality changed completely, becoming a disobedient Pokémon and fighting only when and how he pleased.
Split into simpler sentences: Charmander evolved into Charmeleon during a battle against an army of Exeggutor. His personality changed completely, becoming a disobedient Pokémon and fighting only when and how he pleased.