Crystal Project: All Summon Locations

Having at least one Summoner as the main class and defeating the Deity permanently gives everyone in your party the ability to Learn the Summon skill
The Summon skills will still cost LP to Learn as if they had all prerequisite skills Learned

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

Also feel free to share any other tips because I already defeated them and I don’t remember most of their fights

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 or the crystal with Owl

Just behind Gaea Shrine 

Lake Delende, north of Yamagawa
Just fly southwest from Callisto Shrine
Fighting tip:  

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

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

Shoudu Province, northeast of Ganymede shrine
Good classes to bring: 

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