So these days there are lots of world travel/reincarnation series.
Are there any series about a mecha pilot in a fantasy setting?
Closest I've seen is Gargantia.
I'd like to see a giant robo crushign demons and the like.
Just checked out andm ost aren't what I'm looking for.
I'm looking for worlds where ONLY ONE mecha exists.
Literally WHOA ME AND MY MECH GOT SENT TO SOME TOLKIEN FANTASY WORLD AND ARE LITERALLY SLAUGHTERING MAGICAL ENTITIES LIKE ANTS
If you want one mecha that overpowers everything, you need to drop the fantasy, and watch something like gunbuster.
I think most of the fantasy mecha animes have been named here already.
Very specific criteria you have there. I'm not surprised that charts may fail you
The Giant Robo OVA may be close to what you're looking for.
I've heard Giant Gorg is close as well, although I haven't watched it
Almost all mecha shows have more than one mech, so there's not much I can tell you
So... GATE, I guess, if you count military vehicles as mecha?
Generally the mech is part of the fantasy world and the normal person gets sent there to pilot it. Because, y'know, if they already had a mech then they're not really normal.
There's Knights and Magic, where mechs are common but the reincarnated main character is an insanely skilled pilot and engineer who can make them do impossible things.