Battle Command Repurposing

I'd like to create some "new" battle commands, but even though I've gotten the hang of hex editing, I'm still at a loss when it comes to a lot of the ASM work. I'd like to replace certain abilities which have no use in my hack with one-off abilities which pull from the spell list, the same way Shock and Health do. This seems like it should be relatively easy, but my attempts have only been good for crashing a perfectly good ROM. Has anyone had luck with this sort of thing?