Use the strongest available siege units against metal structures
Focus fire on one metal target at a time
Research and upgrade attack technologies before engaging
Use melee units with high crush damage if available
Bring ranged units to eliminate defenders first
Use siege weapons to break walls and gates
Attack from multiple angles to overwhelm repairs and defenses
Target supporting buildings that produce or repair metal defenses
Use fire-based or anti-structure units if your faction has them
Keep your army protected while the metal target is being destroyed