Borderlands: The Pre-Sequel

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.

If you plan on doing a full Athena playthrough, you should get this without any problem...if not though...

It's a bit situational, but if you meet these conditions this method can get you the achievement in just over a half hour:

Have an online/local friend with a high level character (required) and the quest "Guardian Hunter" active or able to be accepted (unlocked after the final boss and makes XP grinding way easier than just dragging a level 1 Athena into UVHM)

It'll be quickest for you if that character is a Lawbringer because she can torch through the guardians much faster than anyone else thanks to showdown improving gun damage and fire rate drastically, but anyone will work. Personally I had a level 50 Nisha helping. Enter your level 1 Gladiator. Use the XP exploit on the guardians in this quest.

They endlessly spawn from the same point so just spawn camp them with the high level player while you reap the rewards as Athena. You want to get her to level 25. It should take about 10 minutes in regular mode and about 5 minutes in TVHM or UVHM (maybe slightly longer if you aren't using Nisha).

Once that's done go back to regular mode with the high level character alongside you and assign Athena's skill points in the phalanx tree being sure to get 'Stalwart' maxed and 'Wrath of the Goddess'. Invictus helps too.

Go to an early-game area and spam your action skill. While it's active have your friend shoot the crap out of your shield. Thanks to Stalwart, all that damage gets absorbed, meaning throwing the Aspis will one-shot everything in serenity's waste and regolith range and with Wrath of the Goddess, each throw will take out several enemies. You shouldn't need to complete more than those two areas if you use the Aspis throw to get all your kills.

This can also be done on your own with a second controller, but that will require that you've played local co-op enough to have a second profile that's high level and will add a bit of time since you'll have to control both characters.

Don't freak out if you see the "Action Hero" challenge is still at 0. For some reason the Aspis throw kills don't count toward this, but the game keeps a tally of those for your achievement.