Actually, the plate that would have shown Zexion's weapon was broken, which means you can't see the colour of it, and therefore, it's impossible to know if he's dead. But Kingdom Hearts 2 Final Mix+ shows Zexion dyng so yes, he is dead.
I'd totally sacrifice my wrists to play KH 2 Final Mix so you shouldn't be complaining. =P And about the armoured figure; what level are you and which difficulty are you playing on? That sounds like the kind of boss I'd like to play against. D= Also, if you can't get past him (or Marluxia for that matter), may I suggest you go synthesize yourself a bunch of Defense Boosts? I got enough synthesis items for around 10 Defense Boosts and 10 Power Boosts in the original KH 2 in only aroudn 4 or 5 hours and I could have synthesized more if I wouldn't have run out of the crystals that synthesize a higher level item. (Forgot the name. ._. I'm in serious need of more Kingdom Hearts! >O)
If you want to be really lame against Riku, just fill your deck with attack cards with the value 0 and Elixirs/Hi-Potions and just keep breaking his sleights. When I was playing through the game without taking any HP on level ups I did that to Vexen and in the end he had two cards in a sleight and two cards in his deck. Don't ask me why he didn't pick a third card for the sleight. O.o
I'm only at level 63, but that should be more than enough to beat Marluxia's absent silhouette. It's not the lack of HP/defence that kills me, it's more often the fact that my count runs down to zero. :| I gave up for a bit because my hand was seriously doing some weird twitching thing. I'll pick it back up again in a week or two. XD I'd completely forgotten you could synthesize those items though, rofl, so I'll have to remember to pull out my guide and figure out which materials are what. :B
