They stored the harp in a storehouse, surrounded by a dozen Angler Soldiers and four Archangels. The security was strict that even Ameanum would probably die if he tried fighting them. But fighting wasn't in the plan.
Also, sneaking their way in as they used to was not an option since the Archangels could see invisible creatures. Also, their PERCEPTION was even higher than Ameanum's Eagle Eyes. They were foes that he couldn't handle with his skills, but there were other ways.
Shingi started testing the waters as he controlled the Spirit Element of the Tower, closer to the Archangel, but not in a way to affect them. Instead, he wanted to see their reaction, and there was none.