Warhammer 40,000: Kill Team

Chapter One choose the Sternguard Veteran. While Progressing through the level keep the enemy at bay by using your ranged attack with the bolter. If things get dicey and you find yourself crowded make liberal use of your power up as it regenerates with more kills. I found that using the sprint while backing up can get you clear of an orc close combat mob quickly!

Ensure that you use the powerups to your advantage but hold off on the Health ones until you have cleared an area (the game will do a slow kill of the last enemy) or you are around 1/4. Do not be too quick to grab these as there are several points that you are rushed with a large force of orcs/greenskins.

I did this on the first mission. If you die hit pause/start then reload last checkpoint. You will need to redo the last section but you will still get the achievement.

This is best to do in co-op. Note that every time you reach critical damage in co-op, you simply go down and await revival. Although your partner can let you die, as long as they revive you in time you're still fine for the achievement.

Additionally, if both of you are downed at once, you'll revert to the last checkpoint and it won't count as a death - meaning you'll still be able to unlock the achievement. As long as you never get past the 'revival stage', nothing else you do matters. Even your co-op partner can die (meaning you can be a jerk and not revive him in time).

NOTE: Like all other achievements earned in co-op, this will only be awarded to the first player. The second player can die or not, but even if they're untouchable they still will not unlock the achievement.

I also got this one on the first level using the Sternguard Veteran.
His Heavy Bolter is great against ranged enemies and as long as you remember to make liberal use of his special ability any enemies which get close to you will be mowed down under heavy fire.

Also remember that if you pick up a regeneration power up it will continue to heal you while it is still active, so don't go rushing off to pick up a new power up.

Chapter One will be your best bet for this achievement. Personally, I used the Librarian, which is a melee-focused class. The Bolt Gun will assist in keeping the weaker enemies at bay, while also softening up the much tougher enemies as well.
Don't hesitate to whip out the special attack in tough situations, which is a vortex blast that will wipe anyone nearby off the map.

The only real hard part will be getting by the larger hordes of gunner Orks, as they can rip your health down quickly. Luckily, the first stage is relatively easy and health pickups are REALLY plentiful. Just make sure you don't waste them, or the healing potential they grant.
Careful use of the hand grenades is also a must (such as, say, keeping three behind for the final rush), as it can help you out of incredibly sticky situations.

If you have already unlocked perks, adding as many melee damage and health bar modifiers as possible will help IMMENSELY.

Of course, co-op wouldn't hurt either. I'd recommend a combination of Librarian and Tech-Marine for a littering of dead bodies at both close and far range.

this is easier done in coop, let your partner do most of the killing and take the brunt of the enemy coming at you and it should unlock with ease