They had the choice of sneaking in a while under the effect of INVISIBILITY, as they had some scrolls prepared. Still, Shingi was confident that the watches had tools or some ability to watch invisible creatures.
Only he, Hineko, and Litle Phoenix had a high chance to make it through undetectable. Hineko's Skill was relatively high after training with Soryn, that even Shingi could have a problem finding him sometimes. Little Phoenix could cast SHADOW FORM and carefully go through, and so could Shingi with his LIGHT FORM, since his STEALTH was still at Base Rank.
But they couldn't leave the rest of the party behind.
What they needed was a distraction and a pretty big one for this situation.
Shingi told the rest of the party that they should move to the other side of the camp from where he would be and see an opening to use it. He only told An to stay behind.