first you have to do that mission thing with the emperor,somewhere on that mission you
meet that sheep/ram god and get the power to slow down time. after that you go to where
her guards are standing out front of the palace and use that power to slow down time and
run past them before it wheres off

wolf_parade is absolutely right, but...! To make sure, might u be talking about the river
of LAVA inside? Well right after u cure the emperor and saved Kaguya, the Bamboo Girl,
head onwards to Sasa Sanctuary. From there u meet her and Mr. Bamboo, and you have to play
the digging mini-game in order to be rewarded with a special divine artifact. When ur done
and have succeeded, she will give u the Fire Tablet! Equip it, and use it 2 swim thru the
river. =3

The secret lies within the veil of mist technique, received within the emperor's palace
after finding the flask, being sucked in, and blowing the air out with galestorm. When
approaching the guards, draw the veil of mist symbol, and run past the guards before the
technique wears off, or when the circles disappear. Do not linger near the guards, for
when the technique wears off, it prompts the guards to kick you out of the palace grounds.