I have recently discovered that the NeSlug in FFX can be extremely annoying if you're not able to deal magical damage (or use lots of powerful overdrives) extremely quickly. At first it's not too bad: he has some fairly powerful attacks, but if you can handle Earth Eater or Greater Sphere you can deal with them. But once you deal enough damage, he goes into his shell. When he's like this, he's immune to physical attacks and is automatically in Regen status, healing upwards of 40,000 HP a turn, and won't come out until he's completely healed. I suppose he's easy if you know that Overdrives like Blitz Ace and Attack Reels still damage him normally, and have enough people with full overdrive bars and Entrust to be able to use them repeatedly, but if not you're gonna have to do it using magic, which is a pain in the ass unless you have three characters who can deal lots of magical damage extremely quickly.