I would suggest you get to know the game better. There are a number of ways that you can train players. To answer your question, if you have a Legend player, the cards usually have the team that the player plays for (it's located in the bottom right corner of the card). There are cards that can be earned by completing sets that are worth more when training Legends and other event card players. Some of these ways are using other Legend collectibles or 24/7 gold or elite players that play for the same team. I have used this method among others to get some of my legends up to a level 98. I also have a Turducken +1 level up that can be used to level my player up to a 99, but that's just another example.
Hope this helps you and answers your question. Although, i'm guessing you're still going to find something to complain about with this response. Either way, keep grinding... you'll get better!
This is also true for Top 100 players. If you train a Top 100 player with their own Top 100 collectible, the collectible value is worth double the normal amount. As a strategy, it's helpful to hold onto some collectibles/cards for the right timing on when to use them.
