This achievement is for Wilhelm and his action skill, the Wolf, which you get upon reaching level 3.
If you focus strictly on getting this achievement you can earn it in about 1.5-2 hours. So, start up your game and play through the intro mission as quickly as possible. Doing so should get your to level 3 right around your first boss fight. Use Wolf as much as possible after this until you're done with the level.
Now once you get to Serenity's Waste your sole focus is on this achievement. You can run and get the O2 thing if you want, and do the first two side-missions just to get your dude leveled up, but after that it's all about kicking some kraggon ass. From the spawn point in Serenity's Waste (the fast travel machine) run out and then right around the cliff face, toward the lava where all the kraggons gather like damn fools. Whittle down their health as much as possible (especially the full-grown ones) and then unleash your Action Skill. After this just kind of jump around and avoid the suicidal Kraggon Fuses. You'll accumulate something in the realm of 5-10 kills per run this way, especially as you level up and start dumping all your skill points into the leftmost tree, with the skill that boosts Wolf's damage. Once you've cleared out all the kraggons, save and quit, and then repeat.
Like I said, this will get you the achievement in around 1.5-2 hours. Not too shabby, and probably the quickest character based chievo in the game.