My main playthrough was with Claptrap, so this solution is for those that want this achievement as quickly as possible.
In order to get this achievement you need to play as Athena (as indicated by the achievement title, obviously). Once you reach level 3 you will have access to her special move, which is a damage-blocking device called Aspis. Simply put, the more damage you take the more damage you can cause with Aspis once released by pressing LB one more time. She slings the shield like Captain America, so aim it at the closest baddy.
Now when you first get the Aspis skill it will be relatively weak. In order to combat this, make sure you lower enemy health as much as you can before releasing the Aspis at them. Also, only invest your skill points in the leftmost tree, and on the skill, that increases Aspis damage if you want this as quickly as possible.
What I did was play through the game naturally, avoiding all but the first two sidequests and using Aspis at every opportunity. Apart from a few backtracking segments and a few replays killing kraggons, I didn't have to grind much at all. I earned this achievement shortly after arriving at Concordia by using the skill on the guard cannons during the main transmitter quest (before you first can leave Concordia). I had just made it to level 7. That should give you a firm sense of the timeframe for the achievement, at just a few hours.
EDIT: Maliicious writes: [the challenge called "Out With a Bang" which is for kills with explosive damage (what the Aspis does when thrown) will track the kills for you. Now obviously you shouldn't use explosive weapons or grenades if this is the route you choose to go. But if you just start out with a Gladiator you can use that challenge to track your kills.] Silver Salvo has confirmed, so you can indeed track where you're at with this achievement.