*UPDATE* The ironman playlist is not playable now on hardcore and insane due to a lack of population in those playlists, so TC have disabled them. You can only play and get this achievement on public matchmaking on inconceivable ironman. In this case, as I've stated below, use LFG and go on private, it's much easier.
This should be easy for a 5 man group. I strongly suggest you communicate and put a lfg post or search for a session here. Just to note, any ironman playlist is pretty much extinct, so if you go in with randoms, expect a very long search time, and potentially teammates who don't know what they are doing, unless you get lucky.
You can do this achievement on Hardcore, Insane and Inconceivable difficulties. For anyone who doesn't know, Ironman is a mode where you have to go through waves 1-50 without your entire team dying. When teammates die their cog tags cannot be picked up, and can only be revived by a Team Revive. You gain a 5% XP boost for all XP collected in this mode.
I would suggest doing this mode on Hardcore just to get the achievement, but I have completed a run on Inconceivable Ironman as a Scout on Blood Drive with 4 randoms, so with the right strategy, it shouldn't really be a problem.
This is just like playing Hardcore Horde, just if you fail you have to start again from wave 1, and you cannot start from the wave you failed on like usual. If you fail you go to a Wave Failed menu, with "Options" and "Return to Menu" being your only options on Public, and you can add in "Return to Lobby" with Private matches for the host.
1. If you have Team Revive, put it in as a card, just in case it royally hits the fan.
2. Try and get a decent map to do this on. War Machine and Clocktower are easy and quick ones to do, I would stay away from maps like Checkout and Lift, but you can do it on any map you like, the achievement will still pop. Tac-Com has a guide for those just starting, or unsure on what strategy to use, although some maps haven't been done yet, so full credit to them: http://gowtaccom.weebly.com/horde-setup-guides.html
3. General tips for Horde apply here, try to level up your class to at least level 7 before attempting, and your skill cards as well, so at least you have the general note of what to do in that class. All runs should contain at least 1 scout and an engineer, and the other 3 people can cycle towards the other classes.
This class will be the same for many strats. Called Shot, Headshot Damage and Precision Rifle Damage are 3 definite class skills for me, but I've heard of Sniper Strike, Precision Rifle Capacity, Radar Ping, Mark Boost and Magic Bullet be used by me and my friends.
With Called Shot at max at level 6, you get a 40% damage boost on any enemy you mark, as well as 10% more power. Having Mark Boost, or Radar Ping, will give you this damage boost, but it's not of paramount importance you need this in as well.
I'll add in Tac-Com's video on the Sniper as well, but this doesn't include the new Rise of the Horde skills for the Sniper like Magic Bullet, Called Shot, and Steady Hand. 90% of Horde games now will have a Sniper, so this update has done wonders to this class.