Don´t try to use your leveled up save. Even if you die and get your initial equipament back, chances are that you are still too strong to make a lasting combo, as enemies will die too soon.

• KEEP THE INITIAL SWORD!
  
• Feel free to change your armor, but only pick armors that DO NOT increase attack or critical rate chance.

• Pick only Blue Crystals (of The Ancient Guard), the ones that increase defense. DO NOT pick the ones that increase attack or critical rate chance.
  
• Get as much food as you can. Superarmor sake is also good.
  
• DO NOT pick any artifact that increases damage, except the DOUBLE HIT ARTIFACT. This artifact helps a lot to get this achievement, as it holds the combo meter for longer, preventing a combo break, but it is not mandatory.

• DO NOT pick any attack nodes. See the print bellow. The orange "circle" shows the most important nodes, I reccomend getting them first. It is very likely you do not need to upgrade them all.

The secret to reach the 300 combo is not to really make a strong combo. Instead, as you get to the desired screen (See PROGRESSION bellow), spam the weak attack (J, by default), and after about 3 hits, push K (strong attack) once or twice to propel yourself forward and gatter the monsters aggro.

You can't lose the combo, so after going forward hit the closest mob with a weak attack and keep doing it. Use the strong attack again only if there is any red arrow in the screen. You need to have all monsters in your screen.

If you lose the counter, push ESC, go to Menu and try again. Game saves each time you are in a new sreen, so do not walk forward if you have killed all mobs and haven't reached the combo.

• Skip the side quests (boy in the first city, samurai in the second one)

• This boss might be challenging, depending on how much defense you have. Avoid its tornadoes and try to counter it when it flies in your direction.

• When you reach this girl and her cutscene, you are in the right place. The fighting screen just after her is the perfect place to get the achievement, as it has many low damage enemies.

If you fail or break the combo, just push esc, go to main menu and try again. DO NOT walk forward if you have killed all mobs. The second screen after her cutscene can also be used to complete the combo, but after that you will start other area with different enemy spaws, such as spiders, which are combo breakers. So try to keep your feet in this screen, refill your life bar when needed and good lucky!

Thanks for reading, I hope this guide can help you. I would like to show my appreciation to all community members who posted about this achievement in the discussions; I haven't tagged anyone here as there are many small tips from different users scatered there, but I could not have made it whitout their help.