A change in 'exp' gain is needed for the current system put in place.
When tracking a single Hollow the experience gained from defeating the Hollow, is about similar to the amount gained from defeating group Hollows.
For instance in one of my turns (forest fight) against a single Hollow, I gained "376 total experience"
In another turn at the same level, I faced 6 Hollows at once and gained "544 total experience".
I believe if your taking on multiple Hollows at once the experience gained shouldn't be almost similar or less (has occured in some cases) when taking a single Hollow.
Please look into this.
Phantom