Ash had a powerful pokemon in Charizard he left it cause they didn't want him to have an overpowered pokemon constantly traveling in his party. Not to mention Charizard had a chip on it's shoulder since the other Charizard were stronger then him. So to me it's partly Charizard self motivation to become stronger and part Liza training, and part sparring with the other Charizard. Overhead is a move? I thought the moves that were last shown were Steel Wing, Flamethrower, Dragon Breathe, Seismic Toss.
It was an example, regardless if he finished in the top 10 still proves the point a Competitive Battler doesn't have to train to go far in Tournaments IRL such as VGC or on forums. They know the ins and outs of how a pokemon can be utilized and what pokemon work best with each other to benefit their said traits.