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.

Relevant cards