Okay, so the amazingly great tactic that no one could've thought of is that he'll go steal the treasury of the enemy demon lord who has better military forces?
Of course.
I mean, it's not like that demon lord bothers having guards for his treasury, right?
Honestly, this sounds retarded.
"I can't beat your forces, so I'll go steal your stuff that protected by your forces to then use it against your forces!"
Clearly the author isn't very good at making up tactics.