First of many Sealed Temple bosses cleared!
Gil Turtle can appear in two places - the Gil Cave in world 2 and the Sealed Temple. It might be different versions of it in each case? I dunno, but I found it hanging out in a corridor.
Gil Turtle's primary gimmick is its counterattack. Every time you hit it, it counters by attacking two random party members. In addition to doing about a thousand damage, these hits poison and blind their targets. Finally, when it dies, it casts a potentially party-wiping Earthquake (this killed me on my first attempt).
My strategy was to ignore poison, because poison is chump change in FF5, and instead focus on keeping alive and healing blindness. As usual, !Mix carried the boss fight - X-Potion to keep people in good health, Dragon Power to boost their damage, Turtle Soup to reduce his defense by 50%, and Levisalve to get Float on everyone. From there on, I was just using the Hands Trick to reset mirage vests on Lenna and Bartz, boosting Bartz and Faris up to massive levels, then outpace him. With Levisalve applied to everyone, the Earthquake missed, and victory was mine.
Bartz: Thief, !Mug, Hermes Sandals
Lenna: Beastmaster, !Mix, Hermes Sandals
Krile: Chemist, !Mix, Hermes Sandals
Faris: Berserker, Artful Dodger, Hermes Sandals
Bosses cleared: 1/7