Frustrating in the best way. I just can't wrap my head around the last puzzle!
The final puzzle is a completely different beast than the rest, I won't go into details but here's the general thought process I went through, expressed as hints:
1: Pbafvqre lbhe tbny: Znxvat vg gb gur rkvg jvgu gur negvsnpg. Jung'f gur svany fgrc?
2: Gur negvsnpg arrqf gb gryrcbeg ng gur fnzr gvzr nf lbhe cynlre punenpgre. Ubj pna lbh znxr guvf fvghngvba unccra?
3: Lbh arrq nabgure jnl gb gevttre gur gryrcbegre jvgu "." flzoby.
4: Lbh arrq gb erpbafvqre rirel cneg bs gur yriry gb svaq n jnl gb qb guvf, fgnegvat sebz gur evtug fvqr bs gur svany ebbz.
5: Lbh arrq gb gevttre n punva bs gryrcbegngvbaf gb ernpu gur tbny, guhf, rirel cneg bs gur yriry vf vzcbegnag va guvf arj pbagrkg.
I won't go into any more details for now, but this should be a very strong start to solve the final puzzle, if you still want to do it yourself, that is. Good luck!