STAR WARS: Battlefront Classic Collection

This can be achieved offline playing against bots. You can check how many medals you have at anytime by doing the following. Select profile>Main menu>Options>Career stats. To obtain this achievement you must earn 4 Guardian medals.

Or in other words earn 24 points in a single life without dying 4 times.

There are 4 different medal categories

Green: 0-3
Veteran: 4-32
Elite: 32-63
Legendary: 64+

To reach veteran Guardian status you need to get the Guardian medal 4 times.
(Guardian medal - Get 24 points in one life)