it's subjective. if you're talking about pure efficiency, leveling to ~35, feed 1 copy with 3 plat fairies twice to get to 50, cc, level to 40, feed copies with 3 plat fairies until cc70, AW, level to 40, feed copies with 3 plat fairies until AW90.
for a non-impossible setup, I level to 40 then combine 2 copies with 2 fodder fairies (anything that isn't gold+. even gold fairies can be considered fodder fairies if you have 100 and no gold units to level) until 50, CC, level to 40, then back to 2 copies with 2 fodder fairies.
but in the end, it doesn't matter. basically, an event unit copy gives 600 exp (base). how you want to make full use of a unit that gives 600 base exp is up to you. plat armors give 1k base exp and level 20 bronze hashim gives 195 exp, for a comparison.