To get this achievement you need to complete a run on Classic Gamemode with a single hero in your party and to do that you need to make use of a mechanic known as Permadeath! Permadeath permanently removes a hero from your party and with that they won't even appear in the Stats screen at the end of the run! You need to use this to get a single hero at the Stats Screen, like this:



But how exactly you can make use of Permadeath you ask me? There are two methods the CURSE METHOD and the ITEM METHOD! Here i will teach you about both.

• Play on Heaven Difficulty.
  
• Enable Myriad Offers in the Options.

• In the Heaven Difficulty you start with a bunch of Blessings which will make obtaining this achievement MUCH easier.
  
• Myriad Offers will show 50% more options of Modifier(Blessings/Curses) which will make it easier to get the Modifiers you want.

For this method you try to reroll your MODIFIERS until you get offered the Curse PERMADEATH, you will then pick this Curse! Picking Curses in HEAVEN DIFFICULTY gives you more points according to the value of the Curse, meaning that you can pick more Blessings to help you! In this case you'd get 6 more points.



All of your heroes will have the effect of PERMADEATH so make sure to play carefully so you don't accidently kill any of them until the Final FIght

You will then play normally until the Final Fight(Floor 20), make sure to put the Boss in a weak enough state that you can finish him easily,let it slowly kill your team until there's only ONE HERO left! After doing that all you need to do is win the fight!!



Done, now you have one the rarest achievements in the game! It's actuallly a fairly easy achievement, the problem is that what you need to do isn't really clear... There's also the ITEM METHOD but this one can be more grindy and be more difficult.

There are 3 Items that will add the effect of PERMADEATH to the hero that has it equipped:



The one you want get is Glass Helm which is a Tier 1 item meaning that you get early in the run, which makes restarting a run for it is much quicker and while doing so you might even get the PERMADEATH Curse which is a easier way to get this achievement! However there are 51 Tier Items in the game, so to help you obtain this item you should try to get the PERCEPTIVE BLESSING:



This one increases the amount of items shown to you! There are many different levels of this one, but you definitely want the higher tier ones since there are so many Tier 1 items.

After obtaining the item you will keep it unequipped in your inventory until you get to 17th Fight, before starting the fight you will equip it to one of your heroes and let them die during the fight! Repeat the process and in the Final Fight you will have 2 Heroes remaining, equip Glass Helm to one of them and start the fight. Then you get the Boss in a weak state and let the hero with PERMADEATH die and win the fight with the hero remaining.

This one is a bit more tricky but doable, try to pick a item that increases the amount of item slots so you can make the hero left alone strong enough.

With PERMADEATH it's possible to win a run with ZERO HEROES left if you end the Final Fight in a Draw, which is still considered a win, however you will not get the ALONE achievement since it requires ONE HERO not ONE OR LESS which is kinda sad.

I did it in one of my attempts for the achievement and it was actually really funny knowing that i did something more difficult, jokingly calling it the non-existent GONE achievement.



I only have the screenshot in portuguese since at the time there was a bug where history of runs wouldn't show if you changed to a different language, i reported this bug and it was fixed already thankfully. It ended with my last hero and the boss dying of Poison