The same 10* with 22+ dark will bring it to 50 in a single go
The same 10* with a different element will add 10 per one, so 2 will bring it to 48.
Any other 10* will add 1 element regardless if it matches or not, so you would need 22 without 5% boosters (which then will take 3 5%s + 7 10*s to hit 50)
More often or not, the cheapest option is to find the same 10* with the same element to grind elements.
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