You know what? I am guilty of bypassing the team objectives in Halo multiplayer in pursuit of achievements. There is this fuel that takes over me when a map that I need an achievement in comes out of the hopper. While I am doing the team no favors while I stroll around a level not shooting and just heaving grenades trying to get Death from the Grave, for whatever reason, I continue to do it. The same goes for hanging out in the backfield on sandbox during team deathmatch to get a Perfection medal.
 
But others have taken this too far. I want to reach through the internet tubes and strangle someone when I get team killed while I have the hammer on Assembly just so they can have a go at the Hammer Time achievement. The same goes for getting team killed if you have the sword on Heretic. And, I'm talking about multiple team kills per game. What on earth was bungie thinking?  
 
On top of all this, you have to wait for the right map and right gametype for some achievements like getting 3 oddball melee kills on a Legendary map. I have, to this day, never played a game of oddball on a legendary map. I laugh seeing people not return flags so they can camp and wait for a shot at getting flag kills. Or better yet, its just comical seeing someone drive a ghost an entire match to get a splatter spree during a CTF game.  
  
Yes, I like achievements. I go out of my way to get them. And, in most cases I enjoy the added value it delivers to my games. But Halo 3 is an example of how to not do them. These achievements ruin an otherwise excellent multiplayer experience by including them. 

I think they're good in this game because the campaign is pretty lame.
 
Usually when I get team killed with the sword it's because the team killer wants it for himself and is to much of an impatient moron to get it after I die or when it respawns

Eh, I enjoyed getting my S Rank. True story.

The easiest way to do this stuff is to do social matches, as you can usually get into a lobby and jump on the mic with "hey, guys, let's go for some achievements in this round" and people that I've found are usually willing to do it. 
 
Other than that...good luck.  I gave up eventually because I realized how much I disliked Halo 3, but after playing through ODST last night, I kinda want to jump on Halo 3 for some more achievements.  570 points is just a little wussy to me.

Nobody is forcing you to get the achievements, i know i have never actually gone out of my way to get them. Sounds like you just play at too low levels where people are idiots (they are also generally idiots at too high levels though) Although to rub it in everyone else's faces, i got heretic sword spree first game.

All these things will happen naturally over time. People trying to force them only have themselves to blame for being impatient. Only questionable ones are "Mongoose Mowdown" and "Two for One" because those legitimately force you to play in ways you wouldn't normally.

yeah all the achievements can be received well all most all of them playing naturally. but there are a few like the kill someone with the odd ball and zombies that you have to pretty much boost at cause the way the playlist is set up, i once went for an entire month of play without playing odd ball once.

I have most of the achievements for Halo 3 related multiplayer stuff. I just play a lot, that's all it is. Achievements were only annoying back when you could only get them in lone wolves, that, that was fucking dumb.

It really didn't matter to me much at all. Its was nice seeing one pop up during a game and I did go after some of them specifically...god i'll never get mongoose mowdown!! i think if your going to make multiplayer achievements and there is no leveling system the achievements should be weird as hell like mongoose mowdown. They should force you to play in a weird and interesting way, because you are only going to get them once.

Simple way to fix it would be to have permanent achievement playlists surely?

I hate multiplayer achievements in general.