What makes you think it will be a difficult match for Leverkusen? Even if we based it on their spirit to fight the relegation battle, their match history looks bad. Darmstadt failed to win in their last 5 matches or so if I remember correctly[1]. Stats-wise, they have a very low xG while having a relatively high xGA, so I'm not sure where does this narrative comes from. Can they stop Leverkusen or fight for a draw? Absolutely, but the chance is arguably low unless Leverkusen plays terribly or has an off day. They just need a little bit of luck to make sure they can score again since making 28 attempts with 7 shots on target (and 2.9 xG) with no goal is quite unlucky.

[1] https://fbref.com/en/squads/6a6967fc/2023-2024/matchlogs/c20/schedule/Darmstadt-98-Scores-and-Fixtures-Bundesliga