I couldn't help but notice this thread and remember a thread that I made that should help you out.
As long as it's on Padres, it shouldn't matter. You just need to be on Padres so you can access the volcanic material. Try looking at
this thread that I made a while back. If you follow that formula (replacing the gold with magma) then you should be good to go.
Hopefully that can help you out a bit. Since a soul requires three gems, you actually need less than what I showed, but if you're fine with a few extras, go ahead with that method.
Happy potioning!