Update Nov 5, 2019:
The Torch and Tail DLC is available again, not sure for how long it'll be available. Thanks ClockWrkPhantom for the info!Update May 2019:
as mentioned by BigWiIIieStyIe and cptn cookie 1 in the comments, it appears that the Torch and Tail DLC is no longer available. Apparently, they rotate character packs and that pack is not available anymore. I don't know if it will be available again in the future, my guess is maybe, but not for a while.Update March 2019:
as mentioned by BigWiIIieStyIe in the comments, now they give you the two characters if you buy the Torch and Tail DLC:
I had already completed the Trials and needed stars so bought the DLC. It gave me stars instead of the keys along with some card packs, 50 rainbow stars and the two characters. All I had to do was select each once and the achievement unlocked.
I suppose you can still go through all the trials, so I'll just leave that info up for anyone who likes a bit of a challenge :) (or doesn't want to buy the DLC)
At the risk of getting negative votes for posting a solution to an achievement I haven't unlocked yet, I'll share a (hopefully) detailed description of what it is that you have to do to unlock this.
Finally went back to finish this! I'm adding a few screenshots and tidying everything up a bit (same information as before, maybe a couple added details here and there). I'm also adding "spoiler" tags so that the solution is not a wall of text (easier to have only the part you're working on expanded)
To unlock this achievement, you need to equip two new characters, one plant and one zombie (Torchwood and Hover Goat-3000), that are inside two very well-protected chests. The steps below will guide you to get to those two chests.Warning:
Save all the codes you get for beating the challenges to dechiper the final code. (If you don't know what this means, don't worry, read on and you'll get all the info you need). (Thank you Kanchanaburi for the suggestion).
In a nutshell, the steps are the following (expanded below):
1 - Collect 50 rainbow stars (RS)
2 - Use the rainbow stars to open 50-RS chest
3 - Go to the vending machine to open the pack you get from the chest
4 - Repeat steps 1-3 until you receive all 4 color keys (red, green, yellow, blue)
5 - For each key, go to the Trials of Gnomus (ToG) area and use the key to open the corresponding door
6 - Complete the challenge & get a screenshot of the code you get at the end
7 - Go down into the chamber in the center of the ToG area and enter the code you got for beating the challenge into the machine
8 - Activate the machine & beat the color boss
9 - Leave the central chamber and go into the portal that opens up when you beat the boss (where the large gnome statue is).
10 - Screenshot the revised code at the end of the path.
11 - Repeat steps 5-10 for each color
12 - Use all 4 revised codes to form your rainbow code
13 - Go back to the central chamber and enter your rainbow code into the machine
14 - Beat the rainbow boss
15 - Leave the central chamber and walke through the newly opened portal (where the giant statue was)
16 - Use rainbow stars to open both chests to get both characters
17 - Respawn as those 2 characters
18 - Achievement unlocked (you might need to go back to the backyard)
So here we go...
Step 12 - Find your rainbow code
Once you use a key, that portal remains available, and you can open a new one and have more than one unbeaten challenge at a time.
If you beat the challenge, you don't have to beat the boss right away, the code you get can be used later on. You can also use it, quit, and come back later and input it again, it will still work.