Mortals and demigods (what could go wrong)

(Adopted by @Apollo_The_Awesome)
Chiron stares out of the broken window, young demigods are training for the biggest fight of there life (the fight against Gaea). The seven have been gone for weeks , what if they don't survive? Suddenly there's a knock on the door, Rachel runs and green mist comes out:

The seven will need to banish the earth,
They shall need to make friends with mortals,
If they are to survive against her,
All of human kind will sleep,
Expect for the chosen few,
But some will not survive.

Make friends with mortal? Rachel fainted and Chiron leaned her against the chair. He needs to worn the 7!

What could go wrong when demigods need the help of mortals?
(All rights go to Rick Riordan. He owns all the character expect for the mortals (introduced later).)