Magus is incapable of casting any type of Earth, Light, Sonic, Force or Spirit spells, and that's because those types of magic dont' exist in Chrono Trigger. Therefore, he has no defense against them. The barrier change wouldn't work against those types of magic because Magus doesn't know those types of magic. The reason that Magus' elemental-change barrier works is because he knows spells from those five elemental schools.

As for the melee damage issue, Magus only had great defense against Chrono's attacks because his party used physical weapons. Raziel's Reaver is a spirit blade, and therefore doesn't adhere to the same rules as normal swords. Plus, the Soul Reaver is an inherently good weapon. It was forged to be the weapon of the vampire hero, it was forged to protect Nosgoth against the dark forces of the Hylden. Therefore, Magus automatically has a weakness against it.

I don't have a ton of time, so I'll get back to this tomorrow.