Tetris Effect: Connected

For this, I highly recommend going into 'Chill Marathon' mode in Effects mode.
Choose whichever level you like, as that doesn't matter. However, I do recommend having 'traditional colours' for the tetrominoes set in the game options.

Make sure that the piece selection you have set when you start the game is 'Fixed', not Random. This will allow you to get an even distribution of pieces, and disregard bad RNG.

I recommend going for this combo with a four-wide valley, as you can see in my achievement video below:

Sustained comboing is something that I found needed both some setup and also quite a bit of practice.

Some considerations:

1) Pick the Quick Play effect mode, FIXED tetromino order, drop speed 1, and make the lines cleared something higher than 30 (60 or 90 is ok; if the tetromino order is good, you can keep topping out your board and resetting it if you make any mistakes)

2) Leave an well that is four spaces wide somewhere, and build a tower (with absolutely no holes) that is so tall that it is only one or two lines from topping out (a pro strategy seems to be putting the well in the middle, but for our purposes, I'll discuss it as if we were putting the alley at the right side).

This isn't the only way to do this, but what worked for me was making sure that one of my early drops should be the I-tetromino. If it is not, then reset. Drop it horizontally at the bottom, leaving a one-space-wide well at the furthest corner of the void (if you build your tower on the left side, then the one-space-wide well will be all the way at the lower-hand right corner). This is the configuration used in the video in the other solution by Dwaggeinite, reproduced here for completeness in this solution:

https://www.youtube.com/watch?time_continue=1&v=LbQtrgzf...

For newer Tetris players, to have enough facility with the game that you can build your own tower in this way, it is good to practice basic Tetris skills first (keeping your tower flat at the top to reduce specific dependencies on certain tetrominos (such as leaving a huge gap for an I-tetromino while desperately building around it til it comes); looking ahead in your queue; appropriate use of CW/CCW rotation, and the slotting S, Z, and T tetrominos underneath overhangs (including T-spin doubles). A lot of this involves practicing what the Tetris community calls finesse, which you can practice at the following website - it will stop you from executing moves that used a suboptimal number of movements:

https://tetresse.harddrop.com/

This will be sustantially easier if you have a handle on all these things before attempting this achievement. Since Tetris is performed competitively, there are also a lot of good tutorials on YouTube that might be of help to you as you grapple with the game.)

3) Be aware of all the possible combos you could enact, and take your time. You will end up consuming the entire tower before you hit your fifteenth combo. This will require enough facility with Tetris that you have practiced looking ahead in your queue, as to decide whether to hold, and what orientation to pick for your given tetromino. This is the most difficult part to practice, and you can fortunately go to this free website here to practice Tetris combos using the four-wide well:

https://ddrkirby.com/games/4-wide-trainer/4-wide-trainer.htm...

Practice this until you get a good handle on the different possibilities for comboing (especially when you have some options that are better than others). Pay attention to your upcoming tetrominos, too, and then once you've practiced enough to get a handle on this you can start trying it within Tetris Effect unassisted.

I probably attempted this achievement around 70-80 times to no avail before, but after being attentive of these steps I knocked it out on my second try.