From what I understand (using the combat triangle)...

 

If your opponent is using magic, you should wear ranged armour.

 

If your opponent is using ranged, you should wear melee armour.

 

So... If your opponent is using melee, you should wear... magic armour? That doesn't seem right.

The way they restatted armour means that magic armour gives more defence against melee attacks than melee armour. So yeah, you could wear magic armour to get better defense against melee attacks, at the cost of accuracy to ranged/melee attacks of your own if you wear body/leg armour or shields.

With the restating its quite simple and follows the triangle.

 

Generally speaking most monsters conform thusly:

Monsters with a melee weakness use ranged attacks.

Ranged is weak to Melee.

Melee armour will minimise damage they do and maximise your damage from weapons.

 

Monsters with range weakness use magic attacks.

Magic is weak to Ranged.

Ranged armour is gd mage def

 

Monsters with magic weakness use melee.

Melee is weak to mage

mage armour is gd melee def.

^ Generally, that's correct. Some monsters use attacks which target their own weakness though, like Skeletal Wyverns (weak to fire magic) using ranged attacks, which hits magic armour pretty hard.

I can kill a wyvern task with protect range on using sharks and 68 mage in two trips there annoying but not at all that hard i use defensive things on my tab also to help somewhat.