Halo Wars: Definitive Edition

Note: In general, I consider my first base location the bottom of the map for map directions.

Mission 1: Alpha Base
Look Daddy Skull -- Kill 100 grunts
Skull effect: When grunts die, their methane tank always shoots off.
The skull spawns in the area outside the gate into Alpha Base.
This is a VERY annoying Skull to meet the requirements--your AI allies will steal kills before you control them, and the bombing of the Covenant will do it too! The key is to speed ahead with your Warthogs and particularly into the Alpha base, kill the grunts before they airstrike wipes them out.

Mission 2: Relic Approach
Grunt Birthday Party Skull -- Kill 20 Jackal Snipers
Skull effect: Grunts explode into confetti.
Skull spawns behind your base in Alpha Base.

Mission 3: Relic Interior
Cowbell Skull -- Kill 45 Hunters
Skull effect: Destruction physics magnified.
Skull spawns right where Forge and Anders were trapped.
You'll want to farm Hunter kills near the exit, but do NOT send Forge and Anders out the exit or the mission will end, then send a unit back to the location to get the skull.

Mission 4: Arcadia City
Wuv Woo Skull -- Kill 50 Elites
Skull effect: Scarabs shoot rainbow beams made of Pure Love. LOL
Skull spawns on the right side of the map, in a little park at the edge of the map.

Mission 5: Arcadia Outskirts
Fog Skull -- Kill 5 Wraith Tanks
Skull effect: No minimap. Final score will increase.
Skull spawns at the end of the highway overpass in the top left corner of the map with your 1st base location being the bottom of the map.

Mission 6: Dome of Light
Sickness Skull -- Kill 50 Banshees
Skull effect: All player units have 50% fewer hitpoints. Final score will increase.
Skull spawns in the garage that's just to the right of your base at the edge of the map.

Mission 7: Scarab
Rebel Sympathizer Skull -- Kill 10 Locusts
Skull effect: Non-player units get 25% more hitpoints. Final score will increase.
Skull spawns at the northernmost point on the map where ground units can go just to the left of the scarab.

Mission 8: Anders's Signal
Rebel Supporter Skull -- Kill 750 Infection Forms
Skull effect: Non-player units do 50% more damage. Final score will increase.
Skull spawns at the top center of the map, easiest to obtain with a flying unit.
This one takes a WHILE to get. Just keep killing flood, and use the flame-thrower turret upgrades. Hornets are also excellent for killing flood here.

Mission 9: The Flood
Rebel Leader Skull -- Kill 20 Flood Stalks
Skull effect: Non-player units start with extra veterancy. Final score will increase.
Skull spawns just to the right of your main base, near a crashed Pelican. Only flying units or transported ground units can reach it.

Mission 10: Shield World
Catch Skull -- Kill 350 Swarms
Skull effect: All player units recharge unit abilities in half the time. Final score will decrease.
Skull spawns on a little island that ground troops can't reach, to the right of your base. Flying units can reach it.
This one can take a while also--just use Scorpions, and park them near the Flood bases, OR rescue everyone and the Flood bases will start spamming units, just make sure you have enough firepower in the center of the map!

Mission 11: Cleansing
Sugar Cookies Skull -- Destroy 100 Sentinels
Skull effect: All player units have 50% more hitpoints. Final score will decrease.
Skull spawns at the very front of the ship in front of the center--you'll need a hornet to get it.
Upgrade all your turrets with anti-air, and they'll take care of the Sentinels pretty much on their own.

Mission 12: Repairs
Boomstick Skull -- Kill 12 Spirit Transports
Skull effect: 5% chance any non-player unit blows up upon death with explosive area of effect damage. Final score will decrease.
Skull spawns at the front of the ship.
Get lots of Hawks and turrets with anti-air to quickly kill the Spirit transports...

Mission 13: Beachhead
Pain Train Skull -- Kill 10 Bomber Forms
Skull effect: All player units train 50% faster. Final score will decrease.
Skull spawns behind where the flood take out the Pelican at the start.
The Bomber Forms will keep coming across the map from the upper left side. Just wait for enough, or go over there with some Hornets to get them sooner.

Mission 14: Reactor
Bountiful Harvest Skull -- Kill 20 Vampires
Skull effect: Supplies come in 25% faster. Final score will decrease.
Skull spawns to the right of the 2nd Covenant base, the 3rd base you can claim.
The Vampires spawn from the top right side of the map, 2 at a time. Just keep killing them as you play the mission, and eventually you'll get 20. If you park units about half way up the final ramp to the top, you can usually spawn kill the Vampires there!

Mission 15: Escape
Emperor Skull -- Kill 3 Scarabs
Skull effect: All player powers recharge in half the time. Final score will decrease.
Skull spawns in the area behind the big Flood spawn point(the one with giant tentacles coming off it all around).
You need to keep killing the Scarabs to get more to spawn--they will even spawn if there are no Covenant bases!