Reducing framerate can be slightly difficult. Some animations are more stable, like idling, so every nth frame can be used. In this case animation will loose its smoothness but you can do so if this is okay. But uneven animations, like attacks, have long phases of preparing and short phase of attack itself. So with simple frame-skipping it can result loose of crucial frames.
That's why you can reduce frame quantity, but for better result you need to manually pick frames that will keep completeness of animation.
P.S. I'm planning to update Isometric Medusa sprites in different spritesheet sizes and in 16 directions with 22.5 degree step. With stable 8 main directions and better angles with another - extended - 8 directions