Mission 8: Swords of Sanghelios - Mythic Skull “Enemies have increased health.”
Mission 10: Enemy Lines - Catch Skull “Enemies are grenade happy!”
Without leaving the inner passages of the Kraken, fly the Banshee to the left and up. There will be two ceiling alcoves along your path. Don’t go in the one right above the hanger, but rather the one past that instead. Expect the alcove to be really tight, but with just enough room for you to exit the Banshee. The Skull will be found on the floor of the alcove.
Mission 12: Battle of Sunaion - Cowbell Skull “Acceleration from explosions is increased. We need more!”
Mission 13: Genesis - Tilt Skull “Enemy resistances and weaknesses are increased.”
Mission 14: The Breaking - Famine Skull “Weapons drop much less ammo.”
This is hands down the easiest skull to obtain in Halo 5. Progress through the mission until the Warden Eternal appears on an elevated platform. After it is quickly dispatched by Cortana, run past the platform (while staying on ground level) and look for the C-shaped hallway in the back. The Skull will be found in the centre of the hallway.
Mission 15: Guardians - Tough Luck Skull “Enemies always go berserk, always dive out of the way, never flee.”
Congratulations, you've found every Skull in Halo 5: Guardians and unlocked the Gravelord achievement.