Here's an easy and quick guide to unlocking this as well as
Firstly, if you don't want to create a custom map or game type, add me on Xbox Live and access my file share. My Gamertag is Myckel Jay.
The map and game type you are looking for are both called "Put Up Your Goose (20G)"
-- clever, I know.
Start up this map and game type on Halo 2 Anniversary. Now invite a friend to help you out or connect a second controller. Both players will spawn in right next to each other. As soon as you spawn in there will be a Gungoose to either side of you. Hop on and kill the other player with it. They will respawn instantly right in front of you. Just hold down the trigger to get your kills. Stop once you get to 25 kills, then start killing them with melee. After each melee kill, you will need to take a step back so they can respawn. Once you get the 25 melee kills, the game will end and you will have unlocked both achievements. If you do not want to download my map or custom game type,
simply go into Forge Mode and delete everything. Now spawn in two player spawn points, one respawn point and a Gungoose fairly close by. Save the map and start up a Slayer match with 0 respawn time and low player health. It's that simple. Good luck!
P.S. - If you don't already have it, this will also unlock
EDIT: If you see the files aren't on my file share for some reason, please let me know. This happened once before, but they should be live again now.