Oh wow i didn't realize dismissing merc doesn't have any exp penalties o_o
Guess i was lucky, i rushed a bunch of orange merc, got most of them to 2 stars and filled them all to at least lvl 35 for formation bonuses, keeping my main merc bloodhorn as high lvl as possible.
I went with the 'can't go wrong' way because i knew orange merc are what i'll need at the end, and only 2 purple ones (or more if you want to swap stats).
That's why i didn't bother seeing if it's better to level lower star merc because it's too much of a hassle xD
That and now with the new orange merc bonuses, it's pretty damn good.
Oh, but i guess it's mostly due to me being a clean freak and not wanting to have blue or green ones in the army