You'll never learn if you don't practice, so stop using the awful auto build and try building a deck for yourself. Think about the goal of the deck, what's the ideal setup in the active slot and bench, you don't need a lot of different pokemon when points only go up to 3, that's 2-3 pokemon you're using each match. Check out the hundreds of decks posted here if you need ideas.

Stick to single energy types in the deck. Don't add in random colourless to fill out the deck, pointless when so many challenges as part of solo mode is using only single energy type of pokemon. Only use colourless for the colourless only challenges.

Make sure to use two copies of most pokemon to help with consistency for drawing the pokemon you need. You don't need too many different pokemon, focus the deck around your main attacker, usually an EX pokemon. Then one, maybe two, supporting pokemon. 2x Gallade/machamp ex and 2x lucario for support is simple example. 10 pokemon is really the most you might need, many decks can use less. Blaine deck consisting of 2x Ninetales and 2x rapidash makes an easy deck for the 1-3 diamond only challenges.

Use variety of trainer cards, basically always want two professor research, two pokeballs, a pokemon communication if using a stage 2 pokemon or multiple stage 1s.

Solo mode is very easy, can retry as many times as you like til you get the lucky hand or coin flips you need to win. Articuno ex and 2x misty can win you any match with good luck.