I don't think Ash really made any mistakes in that battle, rather he just had incredibly bad luck.
Overall though, Ash's biggest mistake is that he doesn't study. In the original series with the Diglett and Dugtrio, Gary comments about how fast the Diglett move and Ash asks if Gary's been studying and Gary responds he has been studying...unlike Ash. That's basically Ash's main problem. Another mistakes he's made is not taking any Pokemon with him when he visits a new region except for Pikachu. 5/6 of his team have little to no battle experience when captured. Granted, he did learn from this mistake in the Battle Frontier and the Sinnoh League as he started using his old Pokemon, as he defeated Brandon with his original starters (Pikachu, Charizard, Bulbasaur, and Squirtle). However, with this mistake, he does make up for it with quick think while in-battle, and come up with creative strategies.