This post and the next update Castillo’s Only Extreme Sudoku review with a solution of OnEx 304 without trials. Most of the solution path is in AIC building, the most challenging level of Sysudoku Advanced.
The only move prior to AIC Building is this Type 2 unique rectangle, which enables the limited cluster and intersecting XY chain extended in the original post into a decisive coloring network by six trials.
The placement 6r9c1 brings the NWc1 boxline removing 3r2c2.
Sudokuwiki AIC building starts with a possible boomerang that finds an ALS node to get to an AIC almost nice loop(ANL).
The starting links are recycled into a grouped AIC ANL.
The first boomer has an obscure start, but comes back in an XY-chain.
Moving right to left in the South tier, boomer 2 is double grouped. The key is the first group, making the following slink out of r9c2.
Now that first boomerang throw can be recycled, including the same ALS, for an AIC ANL.
The breaking point is the nice loop, which introduces Medusa coloring to supplement Sudokuwiki.
The cluster expands and north tier AIC boomer links with it.
The 7r2c8 removal provides the needed slink for a hidden unique rectangle, Type 2. If 4r1c7 is true, so is 7r6c7 and 4r6c8. The slink then makes 7r1c8 true, and the rectangle’s 4’s and 7’s interchange for a second solution, no matter what happens elsewhere.
It’s been a lot to wade through, but Only Extreme 304 requires a lot more. For next week, I’m sending you on a scavenger hunt. You are to collect, in this order, an ALS X-chain boomer(4r9c6), a grouped AIC ANL(4r7c6), another grouped AIC ANL(8r7c2), an XYchain(2r2c8), an X-chain extended XY boomer(2r9c7), an AIC ANL(7r7c6), a coloring trap(7r8c3), and an ALS AIC boomer (1r7c6), a swordfish(2r1c8), a hidden pair, an XY chain(2r2c1), and after a large cluster expansion, a wrap.
Short of the final advanced coloring, you could just load the starting grid into Sudokuwiki and tap your way through it. If you do that, look up unfamiliar terms in the Guide and compare Sysudoku explanations with Sudokuwiki notes and drawings over this post. The drawings and spotting notes will be in the scavenger awards post next week.