x2Dual Armor
If this Pokémon has any 
Energy attached to it, this Pokémon’s type is both and .
Hurricane KickDoes 30 more damage for each Prize card your opponent has taken.
Can I attach a Shield Energy to Lucario with the "Dual Armor" Ability?
It depends on when the Shield Energy is being attached. If Lucario already has a Metal Energy attached to it Dual Armor makes Lucario both Fighting & Metal types, so you CAN attach Shield Energy at that point. However, if Lucario does not already have a Metal Energy attached or if its Dual Armor ability has been shut off, then you CANNOT attach a Shield Energy to Lucario.
If Lucario with "Dual Armor" Ability has one Basic Metal and one Shield Energy attached, what happens if the Basic Metal Energy card is removed? Does Lucario remain a Metal type or would I have to discard the Shield Energy?
Once Lucario gains Metal typing due to Dual Armor it stays that way as long as a Metal Energy is attached. And Shield Energy provides a Metal Energy as long as it is attached to a Metal type Pokemon. So if only the Shield Energy remains, Lucario stays Metal type and the Shield Energy remains attached.