Okay, let's really deconstruct Magus' abilities, shall we?

In the second battle with Magus (the optional one in the Dark Ages) his strategy changes entirely. During this second battle, Magus' barrier does not work. In fact, Magus casts no magic until he's lost about half of his life. Why? Why would he choose to use a less effective strategy against opponents that have already defeated him once before?

I'll tell you my theory. It's because his barrier doesn't work outside of his castle. While on his own home turf, he has enough power at his command to single-handedly tear holes in time. But, when he gets sucked into a portal sending him 10,000 years into the past, he finds himself stranded. Outside of his domain, he loses a great deal of his power. Considering that Raziel is not fighting Magus in his castle, it could be argued that Magus' barrier doesn't work at all.

Plus, it's also a fact that after Magus joins your party, normal weapons and melee attacks from enemies can do him normal damage. It could be argued that his weapon defenses are also attached to his castle. So, it's very likely that in this fight, Magus doesn't have any special defenses against weapons at all.

A second argument that I could pose is that even if Magus' barrier somehow managed to work in this arena, he's got one major weakness in it. During the first fight with Magus in Chrono Trigger, the player is told by the game which spell type Magus is weak against at any given moment. Considering that Crono and his friends never encountered Magus before, and therefore they didn't know anything about his elemental-change barrier, that obviously means that they can tell what his elemental weakness is just by looking at him. Perhaps his aura glows a certain color or maybe it's something else, but obviosuly Magus has some sort of visible giveaway that alerts enemies as to what his elemental weakness is. Raziel can change Reavers instantly, and therefore he can damage Magus at any time with almost no preparation.

Laser Spin causes "Shadow-type damage." That's Spekkio's exact quote. Robo's shadow-type attacks use MP and mimic magic, Spekkio said it himself. It's a magical attack, the Reaver is not.

The Masamune loses it's advantage over Magus in the second fight because Frog chooses to fight him out of revenge. That's a negative emotion. Raziel has been thrown into this fight against his will, he's not choosing to go up against Magus any more than Magus chose to go up against Raziel. Therefore, Magus is a threat to Raziel's life, and if he's a threat to Raziel's life he's a threat to Raziel restoring balance to Nosgoth. Therefore, Raziel is fighting what he percieves to be an enemy of Nosgoth's restoration and therefore he is fighting Magus with the heroic intention of surviving and continuing on his quest. The Reaver cuts right through Magus' defenses.

Either way, you have no arguments to fight back with. I've responded to everything you've said and Raziel has an airtight defense. Let me summarize this as simply as possible.

1. The Reaver is not magical, therefore Magus' barrier does not absorb it. You said it yourself, Magus' barrier absorbs magical energy, and the Reaver is not magical in nature. Robo's Laser Spin attack is magical, Spekkio says it himself in Chrono Trigger, that's whay Magus can absorb it.

2. Magus' barrier either doesn't work outside of his castle (which is why he can't use it when he joins your party or when you fight him the second time) or if it does Raziel can tell what Magus' elemetal weakness is just by looking at him. Plus, because he's not in his castle, Magus weapon defenses might not work at all, considering that normal melee attacks from enemies do him damage when he joins your party.

3. In this fight, Raziel is using the Reaver to defend himself so that he can continue his quest to restore balance to Nosgoth. He did not willingly pick this fight with Magus, and Magus did not willingly pick this fight with Raziel. Raziel percieves Magus as a threat to his life, and therefore a threat to Nosgoth's balance. Therefore, he is using the Reaver with good intentions, therefore it does normal damage to Magus.

4. Magus has absolutely no defense against TK attacks, therefore his advantage of flight is nullified, The instant he tries to lift off into the air, Raziel will pull him right back down. Plus, Raziel can deal him normal damage any time he wants by simply slamming him through a few feet of concrete headstones, or into a crypt wall.

5. This one is my favorite. Magus has no ability to heal himself whatsoever. His only opportunity to heal himself is if his magic barrier works (which I've already basically proved that it won't in this fight) if Raziel casts a Reaver Spell that he can absorb (the Reaver is not magical, therefore it won't heal Magus) and if Raziel isn't paying any attention to what Magus' elemental weakness is, because obviously, he can tell just by looking at him. Basically, Magus has no chance in Hell of healing himself.

Magus can't possibly win.

Raziel: "Nice scythe. Let's see how it looks imbedded in your skull."