Having at least one Summoner as a main class and defeating the Deity permanently gives everyone in your party the ability to Learn the Summon skill with LP
The first Summon, Pinga, is automatically unlocked without fighting a deity

The Summoner crystal can be found at the end of Slip Glide Ride in Tall, Tall Heights, which requires an Owl

Inside a cave underneath Okimoto N.S., requires Owl or Salmon

South of Old Nan's Watering Hole, Spawning Meadows

Southeast of Tall, Tall Heights, southwest of Chalice of Tar
I recommend you fly there from the top of Castle Ramparts, from Callisto Shrine, with the Owl
You can also technically fly there from Dione Shrine but don't bother

Just behind Gaea Shrine

Lake Delende, north of Yamagawa
Just fly southwest from Callisto Shrine
Fighting tip: Floating will make Ioske (try to) waste a turn removing the buffs
Also apparently there's an Axe that makes you immune to Earth magic permanently?

Middle of Salmon Bay, can be reached from Open Sea, west of Poseidon Shrine and River Cat's Ego, requires Owl or Salmon

Deep Sea, far south from the end of Ancient Reservoir, Sara Sara Beach, requires Salmon

Shoudu Province, northeast of Ganymede shrine
Good classes to bring: Ninja, Warlock, Hunter or anyone who can attack fast

West of Jidamba Tangle, climb the trees or waterfalls to get up there