Efficient enhancing?

I've been using 2*s to enhance into 2*s until level 15 and use it on my 5* units. Both of the 2*s aren't leveled before they are used to enhance Into each other. Is this a good way?

IIrc 2*s should not be levelled above level 4 if you want to stay efficient. So by mergeing 2x 2*s you're done.

3*s can be levelled to level 10 or so with efficiency maintained.

4*s level 15 I think?

But don't take my word for it. The info is available on the forums, get creative with the search function and you will find it.