Warhammer 40,000: Space Marine

Like the above solution, Arena 1 is the best place to get this. 5 waves, no deaths.

We fielded all Assaults with Impenetrable and a mix of Mauls with Killing Blow and chainswords with Swordsmans Zeal. All waves with have the Sanctioned Pyskers but only wave 5 has Marines, 4 in total with bolters and a slow power axe melee attack.

Ground pounds and single hits will kill any of the guardsman and gretchins while Impenetrable all but shields you from the melta gun blasts form the Stormtrooper guardsmen. The Pyskers can be killed with 1 direct hit because of Killing blow or a few indirect hits.
Berserker orks can be killed with 1 direct hit with the maul or 2 indirect but the first hit knocks them down to be finished unopposed. Last are the Space Marines, the bolter variant cannot stand up to most melee attacks and with Swordsmans Zeal, your chainsword friends can take them down without trouble. 2-3 hits for the maul, a couple more for the sword.

Only 2 real dangers are in the first arena.
1 is when the Berserkers spawn. They can and will overwhelm a Sword Assault Marines but Maul users can break through them without their armor being broken.
2 is the dead drops. My advice is do not jump over the chasm, use the bridges and Impenetrable will soak up the bulk of incoming fire until you reach swinging distance.

Can be gained by completing the first 5 waves (or any of the other arenas) of Chaos Invasion without anybody using a life (I say the first 5 as this is what i did and is the easiest). Can be done in a private lobby with any number of players.

A copy of my guide for beating the first 5 waves:

With 4 Players this is pretty much a cake-walk.
Tips:
Power Armour will save you! It has a lot faster regeneration speed in this game mode and so does your health, do not be scared to run off, find cover and regain your health.
Conserve ammo by melee attacking lesser enemies like Gretchin and Guardsmen.
Target Imperial Psykers first as they are the only ones who will do any real damage at range in this arena
You will fight Space Marines in wave 5, they take a fair deal of punishment. I would suggest at least one person brings a Melta Gun to bring down their armour quickly.
It seems the Marines carrying heavy weapons have significatly less health than those carrying melee weapons.

*Any futher tips will be appreciated and added to the list, please leave as comments*

I pulled this one off by myself. Private lobby game as a Raptor(Assault) with swordsman zeal and impenetrable. A little tricky during the point capture but not that hard. Thanks to the above guides for the pointers on prioritizing psychers and marines.