Moonlighter

A fairly simple achievement that you may or may not accomplish on 1-2 playthroughs anyways, especially if you're trying to get the Savage! and 100% achievements. That being said, there is a speedy way to get these types of achievements, especially towards the end-game, when you may only have a few more runs left.

In town, Eris at The Wooden Hat (the alchemist/enchantress) will make potions specific to each dungeon that will guide you to the next floor/floor boss (fireflies will highlight what door to go through). You'll need to use 1 potion for each floor, so ultimately, you'll need 2 potions per time through the dungeon. You can play through the 3rd floor if you'd like, but when I was trying to run through these quickly, I just warped back to town right after reaching the 3rd floor, dropped my loot at the house and ran right back into the dungeon.

You can knock out a floor in less than a minute by just following the potion's fireflies. Each forest dungeon firefly potion cost me 4,749 gold (potions are cheaper at night), so make sure you have plenty of gold to spend, which hopefully won't be a problem at the end-game.