Originally posted by KingD19
He snuck them past Heimdall using pre-existing portals only he knew about. His armor is clearly metal, as it's shiny and you see all the metal on it and it's bulletproof. He's even got a big honking metal helmet. In a fight there's no way for him to bypass Erik's shields.
There's actually no proof that the armor is metal. But even if it was, Loki can magically will his armor to disappear and appea lr as we see him do in Avengers.