It hasn't been a much problem for Liverpool until reaching this phase really. Atalanta can be considered as the biggest threat so far but I don't expect them to be the team to eliminate Liverpool either.
Liverpool's next competitor will be Benfica or Marseille next time in this case. I don't think any of them would be able to beat Liverpool also. I'm also one of the people who want to see a Liverpool - Leverkusen finale. But Leverkusen's job to reach this phase is more difficult compared to Liverpool though. Let's say they got past West Ham their next competitor will be one of Milan and Roma. I wouldn't be surprised to see Leverkusen being eliminated earlier.
Even though Liverpool and Bayer Leverkusen have high hopes of play in the Europa League final, but there will always be tension in every match that will be played during this journey. Because of course, Liverpool is still very focused on the Premier League because the competition for trophies there is also very tight and difficult, but Bayer Leverkusen is of course in a comfortable position at the top of the Bundesliga standings. But even so, I am sure that the teams that will face Bayer Leverkusen in this phase will certainly put in their best efforts to break Bayer Leverkusen's unbeaten record throughout this season. Therefore, I am sure there will always be tension and the difficult competition in the Europa League will remain interesting even though indeed, the biggest candidates are Liverpool and Bayer Leverkusen.