The only advice I can give you for the demo is to attack as much as possible, and use a potion whenever you get low on HP. It IS impossible to die from that boss, so you just have to worry about the random battles and stuff.
If you want the enemy to miss you with an attack, press over onthe D-pad and hit "defend". This will usually make the enemies attacks miss. However, this also means you'll have to wait for your next turn in order to attack. I wouldn't suggest using the defend command unless it's the only way you can possible stay alive.
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