Wine Pool Inebriation
Dragonblood is probably the easiest considering Ji Hanwu owns a dragon and I imagine the blood of a dragon would be required for that transformation. To me I think the Storm evolution would be much better than dragon mainly because Gao Peng is more than capable of capturing a python or some other reptile and then slowly raise it up to be a dragon. I dont think Da Zi needs to be a dragon just so Gao Peng can have one immediately(he will have a dragon eventually).