Came here after 12 Word Searches to see what else this dev can do. While not as tight in a solo experience, I can definitely see how poking each other's minds in a co-op setting would improve things. I beat it in just over two hours with gratuitous skipping around. Here's some advice (anything spoilery will be in rot13, but no answers given):
A: Only the clues with bolded words are tricky (referring to other puzzles), the rest being basic descriptions of the letters.
B: Remember there's a letter missing, and remember you can put a ? in that space to solve it like a normal sudoku.
C: The hints are very helpful. Urcgntba vf na bqq qrfpevcgvba sbe gur frpbaq bar, naq V hfhnyyl qenj guvf funcr jvgu gjb fgebxrf. V jnf tbaan tvir na rkgen uvag nf "Cbvag", ohg gung npghnyyl nccyvrf gb gjb bs gurfr.
D: It is a word search. Qrfcvgr nccrnenaprf, guvf vf abg yvxr gur bgure jbeq frnepurf--"pngrtbel" fvzcyl zrnaf nyy gur jbeqf unir n pbzzba gurzr. Lbh'yy cebonoyl unir gb tbbtyr n srj.
E: If you don't want to print this out, using select tool in your art program of choice and using "select a layer at clicked point" works well here. Gurer ner gjb jbeqf gung tb npebff gur pragre bs gur jubyr pebffjbeq--gjryir yrggref ybat, naq arkg gb rnpu bgure. Gurer ner nyfb znal vfbyngrq yrggref.
F: You might notice some number sets at the bottom look the same. Gurl ner gur fnzr yrggre, naq bayl pbhag bapr sbe rnpu pyhr.
G: It's exactly like Minesweeper. V arire fbyirq gur jubyr guvat--hfr qrqhpgvba naq jung ahzore VQf ner ninvynoyr gb cva qbja gur nafjre.
H: Use the same technique as E for digital play. I skipped this one because I sucked at it.
I: Remember squares are rectangles. I formed a table for each pair of shapes, and marked off which ones were friends. V qba'g xabj vs Tybevn naq Unebyq pna or cvaarq qbja, ohg va gur raq, vg qvqa'g znggre.
J: The ? is an unknown letter. It's a standard Quotefall beside.