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...
Steps 1-4
*** Spoiler - click to reveal ***When you go to the new Trials of Gnomus area, the one that unlocks the Secrets of Secrets achievement, if you explore a bit you'll notice that there are 4 doors, one on each corner. These doors are locked and you need a key of the corresponding color (red, blue, green and yellow) to open it.
To get those keys, you need to collect 50 rainbow stars by either playing the new modes (which are not always on) or trading 7 yellow stars for 1 rainbow star to the seagull in the Town Hall and then use those 50 rainbow stars to open the middle chest that's in the Town Hall as well.
This chest will be available to open only while one of the events that reward rainbow stars is active, and can be opened only once per event. There are three chests that can be opened, one for 10 rainbow stars, one for 30 rainbow stars and the one we're interested in for 50 rainbow stars (which is the max number of stars you can have at any one time). As far as I know, the chests for 10 and 30 rainbow stars do not give the keys you need, while the chest for 50 rainbow stars is very likely to give you a key after the latest update (thank you Sighris for the info).
If you want to know when the next event is going to be active, you can check the official events calendar here: http://www.pvzgw2.com/events-calendar. It is updated every month with the events for that month (it also shows rux appearances, hero showcases and community events).
As CrAzEdCaNuCk18 says, the events seem to go in order Mix, Dino vs Cats, Boss hunt. The rainbow stars will carry over from previous events so I suggest stocking up 50 while on the dinos vs. Cats as it is much more difficult to get stars on boss hunt especially if you are playing with randoms that have no idea what to do.
So, once you get your 50 rainbow stars, go open the middle chest and you'll get some coins and a pack that you will have available at the vending machine. Go to the vending machine to open it (using 
) and see what key you get.
Steps 5-11
*** Spoiler - click to reveal ***After opening the pack, go to the new area with the 4 doors and head to the door whose color corresponds to your key. When you approach it, you'll see a prompt to press 
to unlock it. Do so, and a portal appears. Behind that portal there is a challenge that you have to pass within the given time to get a code. Once you get the code, note it down and leave the challenge area through the portal. If you don't write the code down or take a screenshot (or somehow make sure you'll remember it), you'll have to beat that challenge again to get a new code (or the same code again?). So don't leave until you have that info safely stored somewhere. (thanks capt curlybob for the info).
Super useful tip from tornprince2012:
A quick comment for those who think they're screwed because they have no screenshot of any or all trial codes.
1 - get in the trial chamber, don't activate it.
2 - spawn as Deadbeard or Cactus.
3 - press Y and spawn your drone/parrot pal.
4 - fly to the code you've unlocked earlier and make a screenshot or whatever.
Also, some tips for the challenges, courtesy of tornprince2012:
If I would rate the trials by their difficulty, increasing from the easiest ( keys colour ): Yellow, Green, Red, Blue. Some quick tips for those who can't do what is being shown in videos.
I expect for an average player not more than 10 tries for the first two. Yellow is very self-explanatory. Get a versatile character, the preference for a weapon is yours - you need to hit little targets, remember. Don't forget to reload in the middle ( after 2nd or 3rd gate ).
Green - I would recommend using Imp, and Imp only. Double jumps all the way, straighten yourself while gliding with buttboosters.
Red - you will have to sweat here a little, but again, legit it would be the easiest to do with Imp. Double jump will save your life more than once, trust me.
Blue - this is the worst, unless you have a photographic memory. My best tip ( out of experience ) - get another person with you. Kid, husband/wife, friend - anyone. Ask them for the easy part. Second gate - 2 last symbols, next gates - 3 last symbols. You remember the symbols before those. Will be laughable easy. Don't bother with screenshots/phone photos - this will take too much time, and you're on timer, remember.
As regards the blue challenge, I think this is a very useful method, shared by Mikasz:
Blue one is extremely easy if you know a bit about art of memory. All you have to do is to form a story with each of the symbols in order. Focus on the verbs, they are better remembered than nouns only. Sentences can be completely senseless. For instance, if you get watering can, fence and lawnmower (in that order) then form sentence like: I watered my fence and cut it. If you get lawnmower, mushroom, shoe, flamingo and barrow then go with: 'I cut mushrooms, kicked flamingo and took it away.
Sentence has to be adjusted to your language of course but form something which contains something you can associate with a symbol and you will be fine. Don't think too much about sense of the sentence, you have to keep your rhythm going. The simpler, the better. This way I was able to complete blue trial in three tries.
You'll be back in the new area. Go to the center and jump down the central portal. You'll fall to a room where theres's a machine to enter a code. Step on the slabs following the code you noted down after finishing the challenge (the first symbol you step on will be the uppermost symbol) to summon the boss corresponding to the color you're working on. Beat the boss. Repeat for the other 3 doors.
Below I'm adding some videos I found in YouTube showing strategies for the challenges. I take no credit for those, they are all from YouTube user GermanShu.
Red Door:
Yellow Door:
Green Door:
Blue Door:
After fighting each boss, you'll get a "revised" code, which is basically the same code you had with two of the symbols striken out (or in red, I can't remember). You use those two symbols to form a fifth code (two symbols from each color-boss).
Step 12 - Find your rainbow code
*** Spoiler - click to reveal ***Each code is unique, meaning that only your code will work for you, so you have to find it yourself, no workarounds.
This is the hint you get in your mailbox:
So you have to use your 4 color codes to create the rainbow code. For example, in my case (this code won't work for anyone else) I got this code after beating the yellow challenge:
I entered that code into the machine down the central portal, beat the yellow boss, and got this "revised" code in the newly opened portal (where the gnome statues are):
As you can see, two of the symbols are no longer shown. This means that those two symbols are in the right position for the 5th code that you have to enter after beating all 4 color bosses. In this case, the third symbol in my fifth code will be the lawn mower, and the eighth symbol will be the shovel. You have to do the same for the other three color bosses to get a full 8-symbol code for the rainbow boss.
Steps 13-18
*** Spoiler - click to reveal ***Once you enter the 5th code, you will fight a wave of gnomes and King Gnomus (the rainbow boss). This is tougher than the color bosses, so choose your character wisely. I went in with my vampire sunflower because you recover health when you inflict damage. Her overall health is lower though, so you might prefer to go in as a stronger character. Choose one you feel comfortable with.
Once you beat the rainbow boss, the 5th statue on the throne in the main area is destroyed, revealing a portal where there are two chests, one for plants and one for zombies.
Each of these chests has 5 locks, and you need 15 rainbow stars to open each lock, meaning a total of 75 rainbow stars per chest and a grand total of 150 rainbow stars to open both. Once you open both chests (you can use any character to open the chests), you need to change your characters to Hover Goat-3000 and Torchwwod to unlock the achievement. (thank you ReAl ImPuLsE and smakzillaa for the info). That is, you have to actually use them to get the achievement, opening the chests isn't enough (thank you BegFourMercy for the info).
Also, once you beat the rainbow boss you'll get a message in your mailbox saying that there's a rainbow stars pack available for you to purchase so that you can get the characters a bit faster.
You can buy as many as you need, but buy only one at a time, since you can't carry more than 50 RS at any one time. The packs give you 30 rainbow stars each and cost 225,000 coins so, if you have that kind of coinage you'll have to go back and forth a few times until you open all 10 locks. To get to the chests you'll have to walk, but to go back to the vending machine, you can press back and select "go back to backyard" and, once there, press back again and select "warp to home".
Notes: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.