On the first map, one spawns at (J-11) on top of a ledge. You can easily grab it from the lower floor. Unless your farming for just one eye, I highly suggest taking this alternative:
Required Items:
Norg Shell 3 Mythril Beascoints (First Time getting the Key) 1 Gold Beastcoin (After getting the key already) 1 Silver Beastcoin (to get through the door to get your Sahagin Key 1 Norg Shell
Suggested Items:
Sneak Oil Warp Cudgel/Warp Scroll
Acquire a Sahagin Key and enter the Ornament Door. If you are in a Group, one person needs the Sahagin Key. The dorr will open for the entire group but closes in like 7-10 seconds. Once this door closes, you will not be able to open the door from the inside. Any one shut out will need to get themselves a Sahagin Key. Once behind the door, follow the path across the bridge and into the tunnel. As a level 75WAR/NIN, all mobs in here con'd Decent Challenge so keep that in mind. You will need sneak to get to a nice camp area further inside the tunnel. Behind this Ornament Door and into the tunnel is Mindgazer heaven. That map I linked above. Go to the very last map on that page. This is where Mindgazer pops. This entire map. I counted about 12-15 spawn locs. for this mob behind the Ornament Door. I highly recommend going this route for large parties. This place can be cramped. Very cramped with mobs. You will have to dance around Nightmare Bats (66-69), Delta Sahagins (62-72), and Robber Crabs (62-67). when your done, you can either warp out or walk back to Norg. this area is Linear so its hard to get lost. Just follow the path the only way you can. CAUTION: There is a True Sight Notorious Monster @ (H/I-10) so hug the Right wall of this path as best you can while keeping sneak up.
About these Hecteyes. Unless you will be a backline job, I highly recommend engaging the mob and unlocking from target. Look for a move called Hex Eye. This move absolutly halts all action on you but can be prevented. If you see this move readied by this mob in the chat log, turn and face away from it. This will keep your character in the fight. These things do not link so pull freely. If you pull using any ranged/spells etc., more than likely the first thing the Mindgazer will do is try and get a AM off. I suggest running away and keeping a distance between you and the eyes so that it does not get this magic off. You have plenty of time to move since AM takes like 20-30 seconds for this mob to ready and then actually cast. Once it realizes it failed the AM, it will start moving after you. If the Hecteyes readies AM during the fight, dont worry about it. I found that me and my THF buddy were able to interupt its casting simply by Meleeing it. I got the drop twice for a record of 2/50. I say we because me 75WAR/NIN and 74 THF/NIN were duo'ing these things. Oh and one more thing, Bloody Bolts are your friend.
Nice pet for BST, I used them today farming Nightmare bats in Uleguerand Range, if you use Sic and it don't have full TP it use black magic, this monster is BLM and can use tier 4 spells and Ga like Firaga III, it also use Stun, Bio II, Poisonga II and etc as BLM can use.
Only a few was T to me as BST 71 most was DC but there were a few EM.
The problem is it's not an actual quest that shows up in your chatlog. Regardless, here's the process:
Talk to Gimb in Norg; he's the Galka leaning against a door near the chocobo. He'll mention the Sahagin Key and tell you to ask the sahagins about it. Then head to the Gold Door and go through. Head to the cliff at F-9 and fall; follow the path down there to a door. In this room, there's a Sahagin NPC named Bou the Righteous; talk to him and he'll mention Hurr the Betrayer making keys for the pirates and how angry it makes him. Now head to the Silver Door and head to the north end of it (the room with the ??? for the Utsusemi: Ichi quest). Talk to Hurr the Betrayer here; he'll tell you to bring him a Norg Shell and 3 Mythril Beastcoins. Give him these, and he'll give you a Sahagin Key. I'm told you can't zone during this process, until you get the key. The key is one use only; to get a new one, you have to give Hurr a Gold Beastcoin and a Norg Shell (no need to go through the whole process again; just talk to Hurr again and he'll ask for the coin and shell.