It's not about what you feed it, rather the Slime learns abilities at Lvl 2 and... 7, I think? My preferred strategy is to work primarily on MATK+HP via Living Hearts, Goblin Teeth, etc, (and whatever else is high value, though Avian Feathers can be a tough pick as Aura's AGI is a rather important stat). Also, 1 Mutant Spore will help with the Morph spells if you don't want to spend money on Mana Stones.
Once you get it later on, the Fire/Wind Slime morphs are arguably the best types to have it morphed into overall.