Saying it was easy, but actually putting it into action was much more difficult.
Damien had received an in-depth explanation on the inner workings and layout of the demon city Acier from Demon King Lucius, but this information wasn't enough to guarantee his success.
There were many factors blocking his path forward.
The main one, and the factor that was the most out of his control, was the massive restriction on his teleportation ability.
If he was as he was in the outside world, it would've been a simple matter to teleport into the Demon King's Castle, grab the Demon King, and teleport out, entering the Sanctuary before anyone could regain their senses.
Honestly, thinking about how easy the mission would've been in such a scenario made Damien's head hurt, so he decided to stop thinking about it altogether. What mattered was that such ease was impossible.