Vertical Drop Heroes HD

The easiest way to get this is to play normally until you manage to unlock the 'open all' skill from the merchant that appears during levels. When it's unlocked and you get a character with it you should be able to dash to the end of the first three levels then use unlock all to open the door to the exit. Be sure auto attack is turned off in options so you don't accidentally kill something.
You can use the spare charge on the way to unlock other heroes to kill things to farm a few levels of the way. If the birds that follow you appear, kill them and just do a different floor as pacifist. If you have too much trouble try again after putting more levels into HP in the temple, my level in HP was somewhere in the high teens when I did this.

For this achievement, you will need to perform a Pacifist run in 3 consecutive levels. That means the only buying you can press is during these 3 levels. Not even the boss can be fought.

Character: Any character with the Keymaster trait is essential for this, as you start with 5 keys.

Shrines: Some shrines will reward you with a key and spawn enemies for this. Use them.

Tactics: Keep a keen eye on the level for any keys that dot the level. Grab them if they're not surrounded by enemies.

Boss: You can skip the boss fight, by approaching the lock for the exit and spending 5 keys. So it's essential you get a good run as you may not find enough and need to start over.

I took 4 attempts specifically aiming for this achievement. When I unlock it, I had 3 spare keys.

Take your time with this, as there is no time limit, but watch out for randomly spawning enemies. The runs I failed on I ended up beaten in level 2 due to the ravens as try swarm you and will shred your health bar down quickly. When I beat level 2, I have no ravens spawn in level 2. I have also upgraded my Health via the Apothecary in the Hub.

Any further questions please don't hesitate to ask.

P.S Don't forget to collect all blue heart orbs. They give XP so you can level up. Once you level up, this refills your health bar.

Edit:
According to DrunkLastNight, these levels do not need l be consecutive after all. Also states that Open All skill works well too. If anyone else can verify this it would be beneficial for the guide.