Playing on normal will give advantages to your troops, since they are all inferior, you will have a sweet way to expand in the begginning, also, your chariots are bugged in auto battles, as they tend to overperform, while in a manual battle they wouldnt do that much.
Listen, I'm not doing this for my own benefit. I designed this script, I know how it works, if I wanted to exploit technicalities, I could win easily, but I want to enjoy myself.

This is what happens to auto-resolved battles when you change the battle difficulties:

Low battle difficulty - gives you, the attacker, an unfair advantage
Normal battle difficulty - no advantage given to either side
High battle difficulty - gives your enemy, the defender, and unfair advantage

So I hope you can understand why we will be using Normal battle difficulty. Seeing as most battles will be auto-resolved.