Probably the most difficult achievement for the average player to get, although quite doable with some practice on your favorite character and some investment in powerful runes and gems. For this achievement you may want to invest in stacking damage gems/runes on either a Striker class, or an enchanter. Some of the Guardians are more geared toward single target attacks (such as Gollum) and are probably not the best choice when AOEing down groups of enemies can net you more kills, and also potentially help steal other people's kills. Guardians such as Legolas, Haldir, and Agandaur are all solid DPS choices. I would recommend sticking to high damage guardians such as these, especially on three-lane maps where you can teleport () back to base and heal up at the spawn tower whenever you need to. One-lanes do not have this wonderful bonus.
To possibly help increase your chance of getting this achievement, you can switch your game to gold mode via the settings on the main menu. This removes all DLC (guardians AND maps) and allows you to play with trial players and bots. These games will generally start once there are at least two people in the game, and the rest of the spots will fill in with bots, which can be considerably easier to take out if you play according to how the bots play (generally, very cautious). Also, be sure to take advantage of any bot healers on your team as they can prove to be very useful in keeping you alive, especially Eowyn.