Secret Grade Degenerate Guide
For maximum degeneracy, you can use this guide to brute-force secret grade. On a 20 tall board, you may have to freestyle bag 10, or you can do secret grade in Custom Mode with a 25 tall board, and still get the achievement... Guide inspired by: fivo's doc.
The secret grade build can be split into 9 different smaller builds, each using a full bag and filling 2L of the secret grade build.
A full bag can create 28 blocks (7x4), which accounts for 2L with 1 hole each, and one complete line (9+9+10).
Notes:
The builds shown next are not minimal sets.
They are full sets which show every possible way of building the shape in question.
A minimal set only shows the necessary builds to cover any possible queue.
Note - not every bag is 100% covered. All multiplied out, a full consecutive sequence has a 60.85% success rate.
Tip: If you encounter a bag on which no build is covered, simply do a 35 piece 14L loop on top such as PCO-2nd-DPC (beware holes - try this degen loop if you want something guaranteed). After this loop you should have a refreshed bag to try again.