A simple question: What is the highest level of experience you've ever been able to nurse a unit up to?
Myself: taking a Praetorian and applying your first few Great Generals got me a level 11 unit; be sure to get the Leadership promotion to accelerate leveling as well as maximizing your Withdraw percentage to increase your survival odds.
It helps if you single out a backwards Civ without metals and mob it throughout the game; essentially warring not to conquer cities or reach any military objectives, but warring to maximize the experience level of your favorite unit!
Myself: taking a Praetorian and applying your first few Great Generals got me a level 11 unit; be sure to get the Leadership promotion to accelerate leveling as well as maximizing your Withdraw percentage to increase your survival odds.
It helps if you single out a backwards Civ without metals and mob it throughout the game; essentially warring not to conquer cities or reach any military objectives, but warring to maximize the experience level of your favorite unit!