In order to get this you need to get your squadron level to max (lvl 145) ánd find all the hidden coins.
There is 1 hidden coin in each level, including the ace battles and the 3-1 stealth level 'Lights in the dark'. You will probably have to fight the bosses multiple times to get all their hidden coins, but you can do this on normal difficulty.
For the hidden coin in the stealth level: completely ignore the optional objective to remain unseen. Just fly into every lightbeam you can and kill as many opponents as you can, because one of them will have your hidden skill coin.