All of the characters are amazing in this game and very easy to do this achievement. However if you want the easiest way to do this particular achievement or any of the difficulty ones. Select the dwarf. Give him the best crossbow you can find and put points into the explosive bolt. Pour all of your skill points into dexterity for range combat and fire the explosive bolt shot at any enemy or groups of enemies and they will die in one hit.
This move does incredible damage and can one shot every boss, enemy, or group of enemies in the game. Seriously it is so broken you will most likely feel bored and cheated by the end of the game as nothing will pose a threat to you.
Also as a side note you can actually make the game easier with any character if you wish to devote enough time or grinding. The mission in Rivendale that has you grab the Mithril for the blacksmith who will upgrade your weapons and gear can be used to create ultimate one shot weapons for all characters and armor that doesn't allow you to take damage.
If you have a flash drive or memory card you can host off of that. Have 2 controllers and / or 3 people via xbox live or system link to make it quicker. Player 1 uses the flash drive to load the game, then pulls the memory card so the quest is not saved. Then you choose to upgrade a weapon or armor piece, trade with the next player and have them turn in the quest choosing to upgrade the same item. Have player 2 quit and save with that item in hand so that the item that is upgraded saves with him. Quit, restart from beginning, have player 2 trade the item back, rinse and repeat. You can continue to do this over and over in order to create "God" weapons and armor that never break and do massive damage and perks. You can also choose to do this a much slower way which is to just keep replaying the game over and over in order to keep redoing the quest upgrading the same item each time but is much slower.
If you take the duel weilding path with the Rogue you can use the dupe item exploit to create a second one handed weapon of godly magnitude once you create it.
Each character is very powerful once you max their level and the rogue is capable of just walking through large portions of the level without fighting, the mage is squishy but can be quite a power house if you build her right. I have beaten legendary with every character and quite legit before doing these few tactics but if you are struggling with the game or just want the achievement quickly, or even just want beast characters that can rush you through the game these are the few tactics I found that can make that happen.