>>714630371All skills and passive are farmable, you can just save scum when a demon is near a level up and reset if they don't give you a skill book.
All demon will always give you the same skill books: one active skill (80% the skill they have by default when they join you) and their passive that unlock when they reach max loyalty.
For example, if you want to give phys resist to Mermaid, you can save scum Hayataro or Yoshitsune until they give you the skill book.
If you can't be arsed to do that, you can just do the usual fusion spree at the end of the game to have a Mermaid will all passives, you even get a message from Victor whenever you fuse a demon that have all passives.
Note that you will still need to farm active skills book to replace the 3 default skills she learn.
Incense can be farmed indefinitely once you clear all Aril Rift: each full clear after the first will give you 2 of each incenses.
Alternatively you can just use the DLC.