Legit method: This must be done on a Halo 2 Anniversary map, a splatter is when you kill a enemy with direct impact from a vehicle, they are simple and fun to get. The only maps with vehicles are Remnant(ghost,warthog,gungoose), Bloodline(hornet,ghost,banshee,warthog,gungoose), Stonetown(ghost,warthog,gungoose) and Zenith(only 1 banshee), going about this legit will take time, but can be done in a week if you are lucky. try doing this with warthog with a team mate on the turrent, you will be a heavy force and have a good chance of splattering your enemy, the hornent works wonders if you slam on the enemy from the air, most dont expect it. if you are looking for the quick way keep reading....
best boosting method is to get two teams of 4 search in H2A playlist, the ideal game mode is 1Flag CTF on Remnant, why you ask? well as long as at least 1 player from either team is holding the flag the game can go for hours, so long as the flag never leaves a players hand for more than 5 seconds, any longer and the game will end. you will have more than enough time to get the kills you need. as far as i know this method only works in H2A, Not HCS or H2 Classic. in my boosting session we got all 8 of us this achievement in 2 ½ hours. just take turns, we made it so each player from blue team each got 4 splatters then red team does the same then blue then red....you get the idea, and when 1 players unlocks it have him just die for the others who have not, that way when only 2 players left need it they can unlock it faster. you can use this method with a wide range of MP achievements(killjoys, grenade kills and so on)