On a scale of 1 to 10 the difficulty is about 9 for someone starting ROM hacking.
The weapons are not compressed, they are stored as 3bpp graphics in the ROM. The "swing" of a weapon is an animation and each unique tile is stored once, not like those PNGs. This is a bit of an uncharted territory for me so I'll stop here. The spell animation are similar, there's 1, 2, or 3 script for each spell. This is documented though and the battle animation disassembly is available on the document page of the wiki. You could import in the ROM as 4bpp the animation tiles and create (overwrite) some scripts for the new spell. It's not impossible to do: https://youtu.be/cYo45VCfQ6U
Edit: For the savestate, it is only used for the palette to allow easier editing, only zsnes palette will load in YY-CHR.
The weapons are not compressed, they are stored as 3bpp graphics in the ROM. The "swing" of a weapon is an animation and each unique tile is stored once, not like those PNGs. This is a bit of an uncharted territory for me so I'll stop here. The spell animation are similar, there's 1, 2, or 3 script for each spell. This is documented though and the battle animation disassembly is available on the document page of the wiki. You could import in the ROM as 4bpp the animation tiles and create (overwrite) some scripts for the new spell. It's not impossible to do: https://youtu.be/cYo45VCfQ6U
Edit: For the savestate, it is only used for the palette to allow easier editing, only zsnes palette will load in YY-CHR.