First of all, use more Stage 1 cards than Stage 2 cards, and more Basic cards than Stage 1 cards. For example, 4 Oshawott cards, 2 Dewott cards, and 1 Samurott card. This is also how it usually comes out in a themed-deck, such as the Blue Assault deck. Also, make sure you don't run out of energy cards. Powerful cards like Black Kyurem and Krookadile need a lot of energy cards. If you don't think your deck has enough energy to last through the match, use cards that reuse them, such as Energy Retrieval, or Pokémon with attacks or abilities that reuse Energy Cards. Next, make sure that your deck is balanced. Let's say that there are 10 Basic Pokémon in your deck. There is a good chance that you'll get about 1 Basic card when you start the match. If you have about 20 Energy Cards, then there is a good chance that you'll get about 2 or 3 Energy cards when draw your cards. I know this because I tested it. I hope this helps you in any way.