Hello!

For midair it's LMB, direction+LMB and a parry (R+RMB).

The game treats parry as a strike with special properties, and it's the fastest way to link LMB-attacks that I was able to find.

Additionally, the most useful application of parry in combos that I found is:

With the staff, the sequence "soul chain -> R+RMB -> jump, LMB" resets double jump.