This Seeking Tech guide will explain how you can get the missable Carbuncle summon in Final Fantasy V Pixel Remaster.
You can find him only during your second visit to Castle Exdeath. This is a one-time dungeon as you won’t be able to get back into this location once you beat the main boss.
Once you make your way to the right side of the 10th floor, you should see a diamond-shaped platform with skulls. Your goal is to step on the top-most skull without falling down to the floor below. Follow the video to see the correct steps to take.
During the fight, Carbuncle has reflect applied automatically. To make the battle simpler, use physical attacks and heal when necessary. Your physical attacks may not do much damage at first, but it will do a lot when Carbuncle temporarily changes his form.
After defeating Carbuncle, you will be able to summon him.