Big_Boss: It's a Weapon :)
You simply can't defeat it if you haven't the underwater Materia (and you must use Morph on a specific enemy that looks like a pirate ship to obtain it).
Garugo: Pretty sure P1na's talking about Carry Armor, literally a robot that would pick up party members in its arms. Reno sics him on you while he gets away on the submarine carrying the Huge Materia. For FF7, anyway, no info to determine who in FF8 P1na got stuck on. :P
Emerald Weapon sucks to fight, too, though!
*Edit - fixed the boss' actual name. Whoops*
Yes, that was the one. And I really can't remember who it was on 8, although I haven't really thought about it at all to be honest. But I think I got stuck on Skinner's Anima summon on X. Anything from then on, I never restarted, I saw no reason for it.
As for the weapons, you do have to grind for those. But I personally grinded so bad on the late game, I kicked emerald's butt without the underwater materia and without the fenix-final attack combo. I just limit broke the bastard, over and over again. It was fun. I couldn't beat Ruby without the knights though, which was disappointing (and boring)