I finally went nuclear on Boris's nails. It's just not right, y'all. Normal dogs have cute little nails. People clip them with wee, inexpensive, Petco-available clippers.
Not Boris. His nails kill Petco clippers. His nails laugh in the face of clippers meant for "large breeds." If large means a dachsund, ok. A lab/mastiff mix? No.
I Googled "clippers for a large breed dog," "cutting nails large breed dog," and other things but found nothing useful. Then I decided to ask people who really know what they're talking about.
Mastiff owners. And, sure enough, on a forum devoted to grooming truly, truly large dogs, someone mentioned farriers' tools.
Ah. Now there's an idea I can run with. If it cuts a horse's hooves, it might work on our house-bound pony. Sure enough, Amazon sells nippers and farriers' rasps. These are tools. Serious, not kidding around, 10-14inch long tools. And since the nippers are rated to cut "large nails [made of metal?]" they do ok on Boris.
He tolerates it with enormous grace, probably because he's used to the usual round of cursing and failure. At least this time, when he extends his paw, something actually happens. He even allows me to run his clipped nails across the rasp (filing is so important in a manicure/pedicure, don't you agree?).
Hooray for the internet! Hooray for Mastiffs! Hooray for farriers!