Project Spark

Edit (5th October 2015): following Title Update 16, the original method of unlocking this achievement became invalid, and an alternate method was introduced.

NEW METHOD (after 5th October 2015)

To view the available challenges for any champion:
1. Press the button when in a menu. If you are in a game (Crossroads, an episodic adventure, or user generated content), press the button and then press the button.
2. Select "Challenges"
3. Scroll along the "timed" tab OR the "goals" tab to find the challenges for the Seph champion. There will be a single daily challenge in the timed tab, 7-11 one-time challenges in the goals tab, and 19 "level Seph up" challenges (note: you only see one of these at a time) in the goals tab.

To complete the challenge:
Most are obtainable in a game of Crossroads. If you are struggling, search the user generated worlds with the search string "challenge", and you should be presented with a list of worlds with a continuous stream of 1 strength enemies designed for easy challenge completion.

If you don't know how to complete the challenge, check out this thread on the official Project Spark forums. (NOTE: it may not be 100% accurate any more, but it should give you a general idea of what to do.)

If you still can't work out what to do, try each ability the champion has, and check your percentage progress by viewing your challenges (see above) after each attempt.

If you still can't work out what to do, give up and try again tomorrow with a different challenge.

Note: this method is retroactive. If you had completed 1 Seph challenge before the title update, this achievement will (or should...) unlock when you start the game for the first time after the patch.


ORIGINAL METHOD (no longer valid, but left here for posterity)

Upon loading Project Spark for the first time each day, you'll be greeted by a screen telling you how many Golden Tickets you have earned by logging in, and what this week's Grand Prize is. There will be an option to "Play Now" or "Play Later". Choose the "Play Now" option.

If you decided to "Play Later", you can find Treasure Trove in the Marketplace. At the main menu screen, select "Marketplace". Then select "Treasure Trove", which is near the top right corner of the screen.

To play Treasure Trove, simply select two squares to reveal.

Note:
If you want to earn all but one of the Treasure Trove achievements - including this one - as quickly/efficiently as possible, I recommend saving up your Golden Tickets until you have at least 20. Regular users earn 1 a day, Premium users earn 3 a day. You can also purchase them for real money too. Simply logging into Project Spark earns you your ticket(s), just don't use it/them. You will also need to visit the Treasure Trove board each day for a week. Beginning on Monday and ending on a Sunday, in order to maximize the daily multiplier. Once you have 20 Golden Tickets, and (on a Sunday) the 4.5 multiplier; use them all in one go to guarantee you clear the board. Be sure to use a paper+pen to record which prizes are where as you work through the board.

edit (25th August 2015): updated solution to reflect new board reset days and times.