Leaving one Builder open for wall upgrades is usually inefficient, unless you're at a higher Town Hall level and have excess resources.
Upgrading buildings that take more than 14 days should be completed with a magic item to save Builder time.
Proper resource management is crucial for efficient Builder usage. Step up your upgrades to finish at different times to avoid resource shortage.
Avoid upgrading low return on investment buildings like resource collectors past level 12 for faster overall progression.
Use Builder potions and research potions strategically, especially for Hero upgrades, to maximize Builder efficiency.