Neoverse

Sometimes, when you are moving from battle to battle, there will be a choice of two or three options. One of these is normally an Elite battle, which will have text saying that "this battle will have stronger enemies" effectively. It will also have a green outline (or blue, if it is a High Elite, which is the same thing, but harder still). This is another achievement that will just come from playing the game, as it is relatively common that you have the option to choose an Elite battle, and they often have good mission rewards attached to them (or have missions specifically relating to them, like "Clear 2 Elite battles").