There are two ways to do this. The hard way and the easy way.
I'll start off with the easy way. If you don't have the DLC Challenge pack feel free to skip ahead to the hard way.
Easy Way:
Load up Silent Hall, and as soon as you are in have one overlord grab all the green minions(and no other minions). There are four corners in the map; find the one with the barrels in it and cram yourself between the barrels and the corner. There's actually plenty of room. Put your green minions on a guard marker and move the marker basically into the corner so they stealth. No large enemies will wander back to this corner. Every five minutes rats will spawn near you and the greens will slaughter them. This is the only thing that could screw things up. If something sees your greens killing the rats it may come and attack you. You can probably survive one attack if you're sitting there, but two will do you in. That's where the luck came in.
The hard way:
There is no tried and true sure fire method with the original maps. You're going to be best off playing on Gates of Destruction or Last Stand. Either way it'll be rough in the end. I'd strongly recommend playing this with someone who you can communicate easily with either via voice chat or in the same room since you'll need to be covering each other's backs. If one dies, both die. If you don't have any way to talk to the other person, you're probably just going to be wasting time.
You should be in decent shape for the first 25 minutes or so. That's when things will start to get hairy. What you're going to want to do at that point is try to kill as many enemies as humanly possible. Go balls to the wall to clear out some running room. If you're better than myself and can fight it out to the end, all the more power to you. More likely than not you're going to end up getting overwhelmed in the last few minutes. If things get too desperate begin to run away like crazy, sacrificing minions to restore your health when you come to the health totems.
Any way you cut it this achievement is going to be very hard on the original maps, and any strategy will be dicey at best with a strong dose of luck or skill needed. If you don't succeed, try, try and try again until you have a really solid feel of the enemies that will spawn at their given times.