*Strictly SOLO for the achievement/Bronze medal and not for the Onyx medal* (although I may take on the Onyx challenge and see how it goes)
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Update - Solo completed on difficulties: Casual, Normal.
-Current incomplete progression: Solo on Hardcore.
-Boss updates since writing this guide: Brumaks are very easy, faced them on Hardcore difficulty wave 20.
-Corpser update: It appears that they are now able to follow you up the stairs and into the tower. haven't seen a game patch, maybe they get smarter with difficulty?!
-Fortifications: Added the sentry south of the decoy
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I soloed all 50 waves. It is quite easy yet a challenge as you get higher in waves but not any harder than playing campaign/arcade on insane.
So you don't need a team of 5 in case you are having a hard time completing all 50 waves, don't have the time to commit, or w/e. If you decide to take on the challenge, here is a my guide of how I went about it.
Although you will be playing solo, I would recommend you sign in a 2nd controller. By doing so, you will rack up a ton of ribbons and XP. It also helps in the later waves if you get in a bind to have the 2nd controller revive you should you go down. Another great reason is the idle character is always parked at the back by the freeway which means no locust will spawn behind you and flank you. (Note, I did not use a 2nd controller on all waves, but on most)
Here's what to do:
Map: Gridlock
Difficulty: Casual
Main weapon: Lancer (without question)
Secondary weapon: (w/e you want)
Pistol: Gorgon Pistol w/Super Reload is beast (use w/e you like however)
Mutators: Super Reload/Infinite Ammo (3rd mutator w/e you want) I used No Ammo Pickups strictly to lower a 50% XP loss to 40%.
Fortifications: On the freeway side. Upgrade the 2 barricades by the wall/car and upgrade the decoy, that is all you need. Later on in the waves when you've saved enough cash, buy the Silverback and park it at your base all the way in the back by the freeway. I only used it in case of emergencies, you will not depend on it much.
Waves 1-40 are pretty much cake and you can actually run around the map freely, 41-50 gets tricky, you will encounter a lot of Grinders which can wipe you out quick. On the left hand side of the car (opposite the Barricade) plant a shield whenever you can so you don't get bum rushed.
The only time you will get overwhelmed is if there are locust in the tower directly left of you. Run up quick, take them out, and head back behind the car.
***VERY IMPORTANT*** Always make sure Super Reload is active, it's like shooting balloons, they go down quick even in the later waves when the locust are super buffed. I constantly wasted regular bullets so I could Super Reload as many times as possible. (Infinite Ammo is key)
Boss waves: The easiest bosses I found were the Reavers, Corpsers and surprisingly Gunkers. Berzerkers can be tough later on and I never encountered a Brumack. If you get Berzrkers and can't take them out, just reload the game, it will give you different bosses every time. It is also important to kill the trash first and then take out the bosses last, unless you have one in your face, then take it out.
Reavers and Gunkers go down fast with Super Reload, Corpsers have thick armor so run up to the tower and unload from there, Corpsers can't get through the stairs and when they dig, they just come up in the same spot. If you get a Centepede, use the Silverback, it was my savior. Or...reload the game.
Remember, you don't have to do all 50 waves at once, you can always continue from the last wave you started. Also, try not to quit in later waves of 10 because having the cash to build your barricades helps a lot but it is doable without as I did it quite a few times. As a matter of fact, my last continue was from 47-50. Finally, don't kid yourself, you will probably die quite a bit in later waves (41-50) but trust me, if you've played insane, it's nowhere near as aggravating.
So, if you want a challenge, it's quite fun and it added value to my game.
Have fun and good luck.