Save often, especially if you're on an emulator and can do it whenever. Also, prepare to have a bad time with grinding. It's tedious and sometimes feels strangely unfair. Like 3 criticals in a row from common enemies with no chance to heal. Hence the constant saving.

Also, personal choice, but I don't think the slingshot is as useful as the bat you have beforehand. I tended to miss more than hit, and my criticals were lacking.

Lastly, don't use attack magic when mushroomed. It doesn't end well.