You need to swap leftmost two red gems, purple gem and green gem, so that green gem moves down and a purple one mover right. The chain reaction will destroy following sets:
1) Leftmost green
2) Leftmost purple
3) Orange
4) Blue
5) Yellow
6) Red
7) Rightmost green
Note that leftmost 2 red gems may be of any color but make sure they don't interfere after you twist.
You may use totally different colors of course, the idea of this guide is a layout that guarentees x7 cascade on a stone challenge and can make x7 cascade in challenger's door should you be lucky with countdown gem spawning.

This layout was created purely by me. If there are other guides with this layout, good luck on searching them, because all i found was "be lucky and use color swapping beasts abilities".

Note that guide logo itself is a layout. This is for better visibility of playing field should the guide be lucky to be in top 3. Its the only guide at the moment of writing.

Unlike cascade stone, in challenger's door countdown gems spawned by your foe will quite painfully disrupt your cascade preparations. It is hard to counteract, but there are few tips.
1) Obviously, if the countdown gem is on the surface, move it away from lower part of the field.
2) If the gem cannot be destroyed without breaking your preparations:
2.1) If it is an explosive one, if you can destroy it with gems that you planned to use in cascade, it is a good idea to do so. Better lose a set or two than let it engulf your work completely.
2.2) If it is not an explosive of any kind, move it on the surface and prepare replacement while it is ticking.
2.3) If it is an explosive one and you ABSOLUTELY cannot destroy it,
2.3.1) If it is a horiz/verti lighting gem, move it so it destroys the least prepared gems.
2.3.2) If it is an engulf gem, move it as above as possible if it is possible to move it on row 4 from below and higher. If not, move it left/right so that it explodes least prepared gems.
2.3.3) If it is random lighting gem, before it is to late, trigger your chain for the biggest cascade you can. Chances are, gems falling from above will keep the cascade and award you and award.
3) If your beast have color changing ability gem, try to
3.1) Place it as a rightmost bottommost gem. If at any point you will have to trigger your cascade due to 2.3.3). This will highly increase successful cascade chance.
3.2) Place it as any gem if you do not planning to use that color anywhere else in a cascade. Not recommended as it can disrupt right part of cascade by making one of required gems disappear in another set.
4) Try to choose beast with an ability that destroys one of enemy gems. When you create that gem try to put it as far from your spawwings but make it possible to destroy it quickly.
OR/AND
5) Try to choose beast with no explosive abilities. You don't want to accidentially trigger your own gem to find that all the board is tabula rasa now.
6) Enemies with locking and stoning abilites are forbidden. They WILL screw you no matter what (unless MEGA luck of course).