In the Champions League everything will be much more difficult than what they experienced in the Europa League. They must be ready for all of this, and they can make preparations as quickly as possible. Xabi must ensure that his team is fully ready to face the next season. He also has to ensure that his team's mental condition is in very good condition, because whether they like it or not, the pressure in the Champions League will be much heavier than what they are feeling now.
UCL is the highest level of football competition, surely it will be more difficult than Europa league. Leverkusen won't be the favorite team in UCL, they are only an average team there. Real Madrid and Man City are the favorite teams in UCL, it is interesting to see Leverkusen playing against these teams. It will be a special match if Leverkusen will play against Real Madrid because Xabi Alonso was a Real Madrid player and he is expected to be Real Madrid coach in the future.
Sure, Xabi Alonso must prepare Leverkusen well. Xabi can't make a random experiment anymore, he must ensure to play with the best lineup and the right tactics. In the next season, UCL will use a new format, there will be more matches. Xabi Alonso must rotate the players, he must be ready for the possibility of injured players. I think Leverkusen needs to buy some players before the new season starts.
For the mentality of players, I'm sure Xabi Alonso knows it well. Xabi Alonso is a typical coach who has a good relationship with players and he is a smart coach in motivating the players.