You're overlooking a few key elements, Kuja.

1. Magus has a good advantage with his changing elemental defense, but there's a hole in that plan. That particular trait doesn't include light-based magic, sound-based magic, force-based magic, air-based magic, earth-based magic or spirit-based magic. Only Lightning, Water, Ice, Fire and Shadow. All of these weapons and spells would cut right through Magus' defense.

The Material Reaver (spirit-based, Raziel's standard Reaver)
The Light Reaver (light-based, if powered up it can blind enemies)
The Air Reaver (air/force-based, if powered up it will knock an enemy down on every hit)
The Earth Reaver (it is the second most powerful Reaver that Raziel can use, and if powered up, it can hit multiple targets with a single strike)
The Force Glyph
The Earth Glyph
The Sound Glyph
The Blind Spell (Light Reaver Spell)
The Gust Spell (Air Reaver Spell)
The Quake Spell (Earth Reaver Spell)

The only Reaver that I will not allow is the Spirit Reaver, and that's because it is an end-game weapon, and Raziel would kill Magus in a single hit with it.

2. Magus has an extremely limited amount of MP to use. Darkmatter alone will only last him a few uses and then he'll be up shit-creek.

3. Magus is no slouch with melee combat, but he's nothing special either. His melee attacks do less damage than Chrono's, Ayla's, Robo's and Frog's, meaning that the only two characters that he's technically stronger than are Marle and Lucca. That's not saying much. Raziel specializes in melee combat, and is armed with weaponry and skills that far surpass those of Magus.

With those things in mind (along with the fact that I have beaten Defiance and now have a greater understanding of Raziel's powers) I don't really see how Magus could win.