Even with popular support for the CSU dwindling, it still seems unlikely for the left to capture a significant number of direct mandates (first-vote seats assigned by first-past-the-post voting) in Bavaria due to vote splitting between the three left-leaning parties (the Left Party, the Social Democrats, and the Greens).
If I were responsible for any of the three parties’ Bavarian election strategies, I would propose to the other two to hold a joint primary election in each voting district. This would likely improve the odds for both the Social Democrats and the Greens while not really changing them for the Left Party (while still giving them the benefit of hurting the other end of the political spectrum plus the opportunity to build alliances within their own).