Leverkusen won the title a long time ago and to win the title while they are unbeaten is a huge record for them but they have maintained their unbeaten streak so far. In the last match they played in the Bundesliga, they were not defeated but drew again in favor of VfB Stuttgart. In matches in other leagues, big teams sometimes get beaten by smaller teams and that is normal, but in the Bundesliga they have started the season with a plan where they have no losing record. In the remaining matches of the season, this team may be undefeated and the German league team may secure one more title after being undefeated in the European League.
Actually Leverkusen has by their records become the team with the highest unbeaten streak having stayed unbeaten in over 35 games this season in the bundesliga alone. They even beat all the records Bayern Munich who happens to be the bundesliga giant has kept as it regards staying unbeaten long enough in the a single season and that's actually very impressive and such that's not common I'm the bundesliga.
So far in the bundesliga it's looking most definite that they are most likely going to stay unbeaten the longest and very probably in the last few games they have got, like you mentioned they don't look like they will be allowing the events of other leagues where big teams happen to loose to smaller teams at times happen to them this season in the bundesliga as they want to win the trophy with an unbeaten records as it's obvious that at this point winning the trophy isn't much of a problem but staying unbeaten is more of what they are concentrating on and aiming to achieve by the last game of the season.
The Europa league is supposed to look challenging to them in that they are most definitely not going have it was to maintain the unbeaten record even if they have been able to maintain it to this point but beyond this point i don't know how well they are going to still maintain such and of course for them to be able to still win the league they will have to bear able to stay unbeaten till the finals as that's the easiest and best way to get to the finals well enough.