As the achievement says, you have to endure the whole load of 50 waves of Horde. This can be done on any difficulty with at least 2 players connected. Other players can drop out and the achievement will still count for you, but if they drop in again, the achievement won`t pop for them. As long as you see the whole Horde through 1 to 50, you`re OK. You can die as much as you want, revive, build defenses, these things are not important for the chievo.
To clarify, this achievement has to be done in one sitting.
The best possible combination of classes with at least 2 players is one engineer and one scout, so you can earn money more quickly and have some good bonuses on building your defenses. The best way possible is to find a spot with only 1 entry point, set up your fabricator there and wait long enough to build a machinegun tower, which is ridiculously overpowered. Have your engineer shoot everything in sight with it, reload and repair after every wave with his repair pistol, and scout will get some nice extra money from the dead corpses. The fabricator upgrades over time with every item you buy (repairs do not count), so have a look at it every now and then for better armaments and spend a little extra for a better choice of weapons.
If you are having problems with the boss waves, DO NOT spend your money on defenses over and over again. Wait it out so you will get the helicopter as a boss, which is the easiest one. Defeat all the regular enemies and then shoot it with about 100 bullets from machinegun tower. If you shoot it down before killng all the other enemies, another helicopter will spawn.
BONUS: Here`s a tip for everyone, who wants to give it a shot in SINGLE PLAYER. It is doable, I did it on my 2nd try. Use map Gridlock, Causal difficulty, pick Engineer class (bonus cards help, but are not necessary) and set up your fabricator on one of the road ends. I prefer Warehouse Spawn, but either one works. Dig in and kill everyone in the first 5 waves, so you can buy yourself a nice gun tower, which will mow down every single enemy with a few bullets. Conserve ammo, so you do not have to reload that much. Remember, engineer has a repair pistol in his default loadout, so you can reload the tower anytime. Build also two fences that will slow down your enemies for extra easy time. With every boss wave, wait for the helicopter to spawn (simply die every time it`s not a helo), it almost never shoots past the curved road where your fabricator is and even if it does, it`s super easy to avoid in the Cafe/Hotel buildings for a few seconds, till it circles around. Around the 20th wave, spend some money to get your fabricator to level 2 (you can upgrade the already built tower, too!) and have a smooth sail till the end that is wave 50.
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