I've no doubt that Leverkusen can beat Mainz even though it will be a tough fight for them in the next match, from the 3 matches I mentioned before, I only doubt that Leverkusen can beat Borussia Dortmund, you know that Borussia Dortmund has a good squad, right?
Let's take a look at this,
Leverkusen vs Mainz 1-0 (1 score at home is not a good thing.)
Dortmund vs Leverkusen 2-3 (Leverkusen playing away managed to score 3 winning goals)
Out of the 27 games played, Dortmund have not had the best results when playing away against top teams. Although Dortmund has a good squad, but mentally Dortmund is a bit bad when playing away. Even Mainz once beat Dortmund when Mainz played at home.
Meanwhile, Mainz when playing at home has a great mentality and has beaten popular teams such as Munich, Dortmund and Stuttgart. That's my reason why the doubts come in the Mainz vs Leverkusen match compared to Leverkusen vs Dortmund.