Double Dragon Neon

Better sit down, there's a lot of info to absorb here.

There's no sure fire way to get this done. A lot of this takes practice and skill, but some high level Stances and Sosetsitsu will definitely help. If it's a boss you're having difficulty with, I recommend going back to an easier difficulty and studying their moves so you dodge effectively. Alternatively, a good video will help too.

Three things are required. A good set of tapes, a healthy stock of cash (for lives), and fuzz face later on.

First off, get these mixtapes to max (or as close as you can). A star next the name means it should be a higher priority on your maxing list. The best level to farm Mythril is level 2 on Dragon or Double Dragon Difficulty. Check out GamePro454's solution for the Max Fidelity achievement.

Sosetsitsu:
*Spinkick - This will be your main Sosetsitsu to use. It hits a lot, does good damage, and you are invincible while you use it
Lightning - There's a few places this can come in handy

Stances:
*Training Wheels - Perfect for boss fights, and places where you can you knock enemies into pits
*Magic Gambit - In some areas, using Sosetsitsu's is the best way to go. This reduces the cost by a lot and the regeneration is slightly faster.
Absorb - If you start running low on health with no dragon kick coming up, try using this stance. Somewhat useful on the plant boss (Marian II)
*Weapons Up - With the right weapons this can go a long way. When maxed, this can do the same damage as someone using Power Gambit wielding the same weapon. I recommend using this with the knife, and bat mainly. The life stealing club in stage 9 also works amazingly with this as well.
Balanced: This is good as an all purpose stance. I personally did not use it once as I focused on mainly on Training Wheels, but it's entirely optional if you choose to use it.

Some useful tips:
- When an enemy is stunned, punch them to uppercut, jump up and attack with X, then finally hit them on the ground with Down and X/Y for a decent combo. Works well on enemies that get up quick like Shun (the ninja-like girl in later stages) and her variants.
- Move up and down to get enemies in place while you avoid them
- Don't jump kick the Linda types. They will usually counter with a well placed kick
- If you come across a shop, buy as many lives as needed.
- If you need to recharge your magic, throw on Magic Gambit and wait a bit before moving on.
- Getting a gleam will make your magic restore faster
- Magic Gambit and Spin Kick spam, especially if you have lots of lives. Just die to restore your magic. This helps out so much and is much deadlier if you have gleam going.
- You can get Fuzzface, and then exit back to the chapter select screen when you unlock him. This way you'll keep him until you restart the game again.

And finally some stage/boss tips as well

Stage 1: This level is easy enough as it is. Take out the enemies as you please, this should be fairly easy if you've made it this far already. Spam Spin kick with Magic Gambit or use a weapon with the Weapons Up stance on the first Abobo. For the enemies inside, just knock them into the pit for some easy kills.

Stage 2: Enemies here are easy enough. Use weapons as needed. The shop should be in plain view in this stage (up the first ladder, then left). When you get to the boss, I recommend using the Bat. Go up to him, dodge his first swing, then wail on him with the bat. Rinse repeat, then use Spin kick when the bat gets destroyed. If you bought extra lives, just Spin Kick + Magic Gambit spam him.

Stage 3: Grab the robots for a OHKO and move on, nothing new in this stage. There's a shop if you really need it though. To find it, keep going through the level until you find the 1up hidden in the destructible machine thing. To the left of that there should be yellow/black tape in the background with a Vent. Just knock it off to get in.

Stage 4: Let the airlock do most of the work for you. Find a way to stun them or knock them to the ground and watch as they get sucked out against their will. The outer areas should be simple as well. The geisha enemies can be taken out easily by taking your time and using kicks. Keep a short distance, after they throw the fan just jump kick and combo. Rinse Repeat. Remember to break as many objects as you can to find 1ups.

For the boss, I recommend Magic Gambit or Training Wheels with Spin Kick. Use Absorb if you need to as well if you find your health runs low. His pattern is simple. Start off by picking a side, and when he shows his bike on that side, attack with Spin Kick or regular punches. You can also hit him with spin kick as he drives by which has a chance of stunning him for extra hits which is probably faster... just watch for when he fixes it as he'll hit you by spinning around.
Off of his bike, just watch his pattern. When he jumps to either side of the screen, he'll fire his gun at you. You can gleam off of his single shots as you move closer by dodging and rolling forward. When he jumps behind you, move out of the way as he'll slide.

Stage 5: Enemies start to get tougher here. A useful tip for dodging the Killacopter's missiles is using the Dodge roll. While rolling you're invincible and can roll through missiles. Use the planks as well when you can. I don't have any great tips for battling Shun so use Spin Kick and weapons as needed. Also try not to be in her line of sight from a distance as she can close the distance quick while attacking. To find the shop, a bit after the Killacopter you should come across a ladder that goes down. Go down and go left to find the shop.

Stage 6: Get Fuzzface, he will make your life easier. Head all the way to the right after the Killacopter. When you find the ladder you should see the two grave stones below. Just stand between the pillars and duck a bunch of times.
For the stage itself, use the pits and falling arrows to your advantage. Knock enemies into the pits. Use Spin kick if you have to or just grab them while they're stunned. For the double Abobo, just lure them into the middle where they'll get hit a lot. Throw that bouncing ball thing as well as it does massive damage. When you go up the ladder, take out the enemies first and then go back to take the top path. Take your time and use Axe kicks (Jump and Y) to get you the extra jumping distance. There should be two lives waiting for you if you can make it.

The boss of this stage involves jumping mainly. Training wheels will make life a lot easier here if you think you'll take too many hits. Destroy whatever you can in your path and make your way up. Dragon Swarm might destroy all of the parts too but I haven't tried it yet to confirm. Use jumping axe kicks and running to get up as fast as you can while dodging the Williams. The rest should be easy.

Stage 7: One of the biggest problems in this stage is Roxy. Use Spin Kick or Lightning to take her down. You can also try jumping knees (Dodge then Y button) or one of the jumping kicks. Bimmy and Jammy are just reskinned Abobos. What you used before should work here too.

Stage 8: There's plenty of room to move around on the elevator. And the destructible objects should have a Dragon Kick and a 1up in them. Use the weapons the enemies drop to your advantage. For Marian II, take out one bulb at a time. Start with the one on the right (Shark). The Dinosaur one on the left when it's exposed will make things so much harder as it not only hits harder, but also has attacks that are screen wide.

Make sure you hit the shop before you go fight the boss. When you get to the falling platforms, take the third one down and go right to find the shop.

Stage 9: One of the toughest stages out there. For the first section, Spin Kick and Lightning spam will help. Use the weapon stance when you get a bat or whip. The whip works well against Roxy enemies and the bat is good for just about all other enemies. If you let Roxy on the ground from the right side, let her dash to the left to automatically kill her. Take out the Abobo in whatever way you prefer. It's likely that you'll end up wasting two of your lives (like me), so don't fret. That's to be expected. The rest of the stage is a lot easier.

For the second part, start off by heading left and breaking the statues for two 1ups. For the Jiang Shi, use either a Weapon Up + Weapon combo or Magic Gambit + Spin Kick combo (or whatever you prefer). Most of these enemies are immune to stun so be careful with how you approach them. If you come across the Healing bat, definitely use the weapon stance. Just keep smacking away until the enemy dies or the bat breaks. Use the spinning spike cylinders to your advantage. Knock the enemies into there for massive damage or use the planks when available. Towards the final part you will have 2 planks and 2 cylinders. Knock the enemies up with the planks and be careful with spin kick... you can take lethal damage if you're not careful due to the elevation. If you're having trouble getting enemies lined up, just move up and down while barely moving to the side.

Stage 10: The final stage! The Bao Boshi is a lethal enemy to face. It's best to corner it and combo it to death using Punches, Kicks and special moves. Use the Bat or Whip with Weapons up to keep it from attacking. Towards the end you'll find some platforms that fall. The third one should have dragon statues and an arrow in the background pointing downwards. Take the platform down and go to the right for the final shop and then continue on.

For the section with 2 pits in the middle, just knock most of them into the pit. Most of the enemies won't move from their spot once they get near the pit. Lightning spam the Roxy enemies to death. Also watch out for the Williams enemy-type as they will pull out a gun to shoot you. After finishing this area and entering the next, go the left and break a pot for an extra life. Continue on and eventually you'll find the boss.

Skullmageddon and Marian are a tough combo. Hit him with your Spin Kick as much as you can. Be sure to use Magic Gambit (or if you have it, balance works okay as well). Switch off to Training Wheels as needed when you run out of magic or you think you'll get hit. Watch out for when Marian starts charging. Skullmageddon will usually have green fire around himself while Marian charges her ultimate attack, so make sure to attack Marian before she can unleash it. When skullmageddon is hidden and the middle and Marian is her powers to suck you in there, you can dispel it if manage to hit her. Generally a jumping spin kick near her does the trick.

Okay, if you've managed to defeat the duo, then you're on the final part. Start by heading all the way left and then down into the corner for some goodies (Credit to Monoxide Mike for pointing this out). Head on over to the boss afterwards.

Giga Skullmageddon is tough. Some of his attacks are the same while other are entirely different. Use your Special move as much as you can and then focus on dodging his attacks while waiting for a chance to counter. His face off attack now sends many more and spread out. Just duck and roll to get a gleam and move closer. Other attacks just require moving around. Watch out for when he moves offscreen and tries to converge some lights on you as this is a OHKO. The achievement should pop a little after Marian's uppercut.

Having personally gotten the achievement, I did not need to use any mixtapes that weren't starred (except for Lightning). Most of the game was done using Training Wheels and Spin Kick. By stage 9 it was almost required that I use Magic Gambit and Weapons Up tapes instead.

Good Luck achievement hunters!

Below I've left a playthrough of someone on single player DD difficulty as a reference. Some of these tips are thanks to the player in this video while the rest are from my own experience. Hopefully it helps some of those having troubles. Whew, that was long.

I don't really have a guide since someone made a good guide for the finishing game on Co-op on Double Dragon Difficulty but I wanted to take the time to remind or tell people who don't know that there is a secret ladder on the last level, just keep running all the way left and hold down till you find the it then climb down and it contains the following.

3 Chests with 2 Keys
3 Lives

So that should help out for the end.
Other than that just max out everything and you shouldn't have an issue except for any ultimate moves thrown your way by the boss.