Should you place guards in all mines? Your army isn't infinite, so you shouldn't waste it like that. And if a level 6 hero turns out to be stronger, you'll lose both the army and the mine, and you'll become weaker yourself.
Use the same tactic. Or place your level 6 hero near the border, give him ten archers, and everything will be fine! Level 6 heroes usually travel with 1 low-level unit.
