This achievement can be quickly achieved with a second a controller.
1. Select Courtyard as the Forklift is located on the furthest, northern point of the map
2. Select the difficulty to easy so the forklift doesn't blow up quickly
3. Go to Options > Spartan Settings >Base Traits > Weapons and Damage, and from there - set the damage modifier to 300%.
4. Back to Spartan Settings, now go to Shields and Health, and set Damage Resistance to Invulnerable.
5. Go back to Options, go to Elite Settings > Shields and Health, and from there - set the Shield Multiplier & Health Recharge rate to 0%
5. Start the game, head to the forklift with your main, and position your second controller within an open-space area of the map
6. Once positioned, both players must walk/drive towards each other, and boom - you've splattered someone with a forklift!
If you're confused on how to pull it off; here's a quick demonstration on how to do it:EDIT:
Forgot to mention a few things, thanks to the replies.
- Don't forget to set both players on the opposing team; 1 red, 1 blue. To be safe, the player on red team will splatter the second player on blue team.
- If doing this through Xbox Live; only the host of the session can unlock this achievement.