Warhammer 40,000: Dawn of War II - Retribution

You need:

• Two friends (hardly you'll convince randoms to help you, neither they have the build you need),
• one of them has to be Lord General with unlocked "Take Aim" ability,
• other guy could use The Sorcerer, because he is squishy and better if using Icon of Pain, which grants nearby allies 30% damage bonus,
• equipped AA missiles, and the main guns probably the best using double plasma gun combo, because it fires fast, and that's what you need.

Sorcerer isn't required to be nearby, but it wouldn't hurt. Lord General is all you need. Your friend probably should evade equipping a sniper rifle, to prevent you being one-shoted by his clone. Maybe you will need to babysit them for the entire round, if they equip weaker stuff. You will know where the wave 16 is by looking at the wave 9, from where Banshees are pouring on there your clones will appear. Stand at the gate with your LG friend and wait for the Sorcerer to start finishing off the last Ork at the wave 15. Then your friend has to boost you with "Take Aim" ability, which will grant you significant damage bonus. A single hit from AA missiles will one-shot your clone, and the double plasma combo will do the short work with weak armour of LG and Sorcerer clones.

Worked in the first try after many failures. In my case, AA missiles shot killed my clone and the Sorcerer who was standing too close to him, and the double plasma just mowed LG. It was over in a blink.

You can either try to search at the same time as your friend, or you can do as I did - ask the guy who is matched with you to play annihilation, and not cap any VPs (also, pray to The Emperor he won't leave the game as you are slicing his base). Worked in the first try for me, gotta say that I was very lucky. Nowadays it really shouldn't be too hard to get matched with your friend and do it the easy way, since the game is pretty much dead.

You can obtain this achievement only on the Anvil of Khorne map, since its the only map having super-heavy units, which are the final tier units from the MP. Those are: Swarmlord, Avatar, The Great Unclean One, Battlewagon, Baneblade, and finally Land Raider.

Regarding their limited number it will take several playthroughs to obtain it, make sure you deal the final blow to them, or ask the peeps you got matched with to let you kill them.

If you are lucky you may get this accidentally when the Tyrant unleashes its RNG attack that damages everything around it. However, you don't have to wait since you can get it easy by yourself. Equip Poison Cysts, which is a starting armor equipment, and play The Anvil of Khorne map. On the wave 6, you'll be attacked by a bunch of Imperial Guardsmen. Don't attack them or use your minions against them, just walk into a bunch, release the poison cloud granted by that piece of equipment, and walk away. Watch Guardsmen being melt equally by the poison, all dying at the same time and granting you your achievement.

This achievement is actually quite easy, and certainly if you have a couple of friends to help you. Shouldn't be too hard to find some, this DLC is free for the taking for the first 5 days of its release.

Anyways, first of all you need to reach lvl 10 to unlock Raiment of Resurrection, which grants you the Resurrection Orb ability. By that time you'll unlock some nasty armor wargear as well. Go all tanky to survive the damage, and your friends should strip of all armor they have. Reaching the wave 3 is sufficient, those Eldars will do the short work with them. Once they are both down near each other throw the Resurrection Orb at them, it takes several seconds to activate after which it will revive them both.

They need to die once more for you to repeat the procedure, after that the achievement will be yours.