The one and only is obtained by selecting the mirror match option at the armax arsenal arena and successfully completing the match on normal or higher difficulty. Set the difficulty to narrative, complete the first and How to unlock the the one and only achievement in mass effect legendary edition
One and Only. Login
It's the one where you need to do a mirror match on normal, hardcore, or insanity
I've heard that using the pain train map and hiding in a certain corner is a decent strategy, as is setting the difficulty to narrative and only turning it back one there's only one enemy left
Also, apparently the modifier that increases player damage doesn't count against you either The one and only ! Here is how your screen should look like for get this achievement. One way to obtain this achievement is by switching the difficulty before the end of the final round
