Your opponent cannot activate “Monster Reborn” and other cards that target while you have a monster equipped with “Metalsilver Armor”. Your opponent can activate “Raigeki”, “Mirror Force”, and other cards that do not target.
If you attack with “Dark Magician”, and you control another monster equipped with “Metalsilver Armor”, your opponent cannot activate “Magic Cylinder” because “Magic Cylinder” targets and only the monster equipped with “Metalsilver Armor” can be targeted (and it is not attacking).
If you have “Metalsilver Armor” active, your opponent cannot target his/her monsters either (with Equip Spell Cards, etc.), including with “Premature Burial”.
If you have 2 monsters equipped with “Metalsilver Armor”, your opponent cannot activate any effects that target 1 monster.
When you have a monster equipped with “Metalsilver Armor”, your opponent CAN activate the effect of “Nimble Momonga” or “Giant Germ” because their effects do not target 1 card.
When you have a monster equipped with “Metalsilver Armor”, your opponent CAN activate “Reinforcement of the Army”.
When you have a monster equipped with “Metalsilver Armor”, your opponent CAN activate the effect of “The Thing in the Crater” because it Special Summons a monster from your HAND.
Netrep Rulings:
Continuous targeting Spell and Trap cards that affect a monster before “Metalsilver Armor” is equipped to another monster remain in effect on the original target.