You must be level 15 to get to the next floor across the entire dungeon.
You need to complete 5 pages and defeat the boss to successfully raid the dungeon.

Smeargle
Jangmo-o
Cranidos
Natu
Rufflet

CHIEF OF THE TRIBE, SMEARGLE - LEVEL 20

If you beat the boss in a fair match and are nice to him, he'll lead you through a secret passage to Radiant Cavern where the level requirements there will drop by 5 for all but the boss.

If you're mean to him throughout the match, he'll call upon Smeargle friends for a round two and the level requirement to beat him jumps to 25. If you still manage to beat him, he'll admit defeat and pan 200 gold out of the river for you whether it's to acknowledge your strength or get you to go away. This gold is on top of the base for defeating the dungeon.