You have to complete four rooms in the Lost Temple to get this achievement. I'll name the room, give any specific items you need to complete the room, a brief description, and some screenshots.
If you need help finding specific items, please look at my other guide:Solution for COLLECTOR in MinitLost Temple North - Basement Key
To clear Lost Temple North you have to SPAWN from a house and die in the room, then you have to do that for each of the four houses. The only way I know how to do this is to use the warps in the basements. So go to a house, get the notice it's your new home, die, respawn, travel to the Lost Temple North, and die again. Make sure that you don't accidentally get reset to a different house when you are warping.
When you successfully do this, another group of trees grows depending on which area you died from. After you have done this from all four houses, the door will be destroyed and you can get a coin (which is technically in the basement of the Sneaker Hut).
Lost Temple East - Watering Can
If you use the Watering Can on the fire in Lost Temple East, you will enter a room full of fire and bats. The goal is to make it across the room with full health. If you do that, the door will be open and you can find a chest with a coin. If you get hit, you'll have to start over on a new respawn.
Lost Temple South - Fast Shoes
Lost Temple South is a really long maze. The only way to get through it is to rush through with the Fast Shoes. Even with the Fast Shoes you'll barely make it so you'll have to avoid hitting as many walls as possible so you can keep charging.
Lost Temple West - 8 Tentacles
This is a gift from the Octopus for finding all 8 of his Tentacles.
If you need help finding all the tentacles, please check out my other guide:Solution for MARINE BIOLOGIST in Minit