In the unlikely event you didn't know, because I myself didn't know for the longest time, you can turn off your Pokemon's shiny particles to slightly decrease the time between encounters.
His account was created in December 2020 according to the forum.
In about 2 years it seems he has dedicated about 246 full days to this game. I'm not trying to hate or anything but please remember this is just a game.
Can I ask how much you play? One day I got bored and mathed out how long it would take to get 28 single encounters shinies at odds and it came out to something like 3 full months of gametime for this single hunt...
Oh boy I can't wait to do the same elfbot dungeon for several hours at a time to try and get a cartoony vanity that I'll never wear.
Woo Christmas "Event"....
I don't think there's any world where a Teddiursa with Hyper Voice can OHKO Mantine hordes 100% of the time, considering they can get up to level 50 or so IIRC.
My easy solution to that is an Elecroweb Compoundeyes Galvantula that I use for my gym reruns.