The Lord of the Rings: Conquest

Just a little add-on to Seraphims guide, you can do this in multiplayer splitscreen as well. You'll just need two controllers. Set the map to Mount Doom, as it has Legolas and Gimli. Set the AI units to off, so you won't have any interference. Set the game to Hero TDM, and run around killing your second controller twenty times, and another twenty for Legolas.

This can actually be done in single player.

To do this in single player, choose instant action and select a Hero Team DM map. It'll say if it has it under the Mode select menu. Not all of them have Gimli however. I would suggest the Shire map as it also has Legolas allowing you to get You Have My Bow achievement if you manage to get this achievement fast enough.

Once you have the map selected, you'll want to hit X and make sure the Team Deathmatch Victory Points is set to 100. If it isn't, you may not get an opportunity to get this achievement before the match is over. Make sure to hit A to save the game settings after confirming the setup.

Go ahead and start the match now. Select Gimli as your character and run off and get kills. Try to stay near some of your allies so you don't get completely swamped. This may also give you some chances to steal some kills (and get them stolen, but what can you do). Again, if you get this achievement and still have 30 or so kills left before the match ends, you can switch over to Legolas and try for the You Have My Bow achievement.