Although some luck is needed to get a chain of 12 or better, you can improve your odds of doing so. Start a run on your favorite rank. Grind your inventory is filled with one "Wood to Key" or "Stone to Key" scroll and two (three if upgraded) keys. Key scrolls are needed because you do not normally spend key pieces regularly. Using key pieces for this will not impact your ability to progress as much as weapons. Die and start a Rookie rank run. During game play, focus on keeping keys and your scrolls' conversion resource on the board. The keys you collect will help by letting pass through a few chests or doors. Once you are satisfied with the board, use the scroll. You start with a few matches and the amount of removal will clear the board, increasing the chance for cascade matches. Die and repeat as many times as needed.
The Treant will first appear as a Boss at Questor rank. Progress through the game and upgrade until you are able to start a Questor rank run and reach the Treant without failing. You will first need to grind Questor or lower rank runs to get a full inventory of physical attack items such as axes and spears. Then, start a Questor rank run and save your items. Once you reach the Treant, use all the items in rapid succession to defeat it.