For those who don't know, an Environmentalist medal is awarded for killing an enemy with a piece of the environment. The map that's gonna get you this achievement is Lockdown, which has 3 large hanging icicles, one located above each base, and one located above the center platform. By shooting these, they will fall and kill any thing they land on.
Your best opportunity to get these medals is at the beginning of a team game on Lockout. When you first spawn, shoot at the icicle hanging above the opposing team's spawn. There's a good chance you can get at least one Environmentalist medal by doing this.
You can also use the other two icicles, assuming they don't get shot down by others. Just wait until you see somebody walk underneath it (which is almost guaranteed to happen at some point in a game) and shoot the icicle above them.
In Assault games, the bomb carriers are more than likely to take the path that leads directly under a base's icicle, so if you're playing a game of assault, keep track of the enemy bomb carriers and try to get your timing right. The icicles will also respawn at the end of each round of Assault, so you can use this tactic more than once in a game.
King of the Hill and Oddball are also great games to get these medals in. At the start of a game, the hill/oddball will be in the dead center of the map, directly under the middle icicle. As the other players race towards the center of the map, time your shot at the icicle so that you can take out large groups of them. Also, if playing Oddball, after destroying the center icicle, try to get the ball and drop it directly under an icicle near one of the bases. The opposing players will likely go for it, and this would be a good time to take them out for the medal.
And of course, there's the boosting method. Get a couple friends together or set up a boosting session on TA. If you can, try to get teamed, slayer gametypes on Lockout. The reason you want it to be teamed, is because you can get the 4 members of the opposite team under an icicle, and get 4 Environmentalist medals as well as an Extermination, which'll count towards this achievement.
You'll need to do this 25 times to get the full 100 Environmentalist medals, and you can do it a maximum of 3 times per round. So, if you're playing a game with rounds, after all 3 icicles have been shot down, end the round by scoring.
The reason that slayer is preferred however, is because, not only will you get 12 Environmentalist medals, but you can also get a Perfection, an Unfrigginbelievable and a Steaktacular, each of which counts towards the Domination achievement. So using the boosting method, you should be able to make 12% progress on this achievement per game, and 15% progress on the Domination achievement (assuming you get the full 12 Exterminations in the game) If you are able to meet all these perks, you could likely get these two achievements in an hour or two. Then of course, get it for the other 7 members of the session.
But, if you'd like to focus strictly on this achievement and not on the Domination achievement as well, try to get One Bomb Assault on Lockout. After all 3 icicles are used to get your 12 Environmentalist medals, you can score the bomb, therefore ending the round, and the 3 icicles will respawn at the beginning of the next round. Since it's 4 rounds, you should be able to get 48 Environmentalist medals, almost half of what you need in total!
I hope this helps. :)