If you can't figure out how to get the points, go to the place where to turn on the skulls. Go to the top bar and change it to (I forgot what one of them is but the other is free for all, doesn't matter which one) then just go on your regular killing spree. Skull also give bonuses, but try not to use all of them, because you don't need all of them. Iron is going to be a pain. Try Catch, and Grunt Birthday Party.
Iron: 3x Teammate dead? Reload checkpoint. Everyone dead? Restart level.
Black Eye: 1.5x No auto-shield regeneration, melee attacks cause regeneration.
Tough Luck: 1.5x Enemies attain ninja-esque dodging, for grenades, Needlers, slow moving projectiles (ie. Rocket launcher, Brute Shot, and Fuel Rod Cannon and vehicular assaults). They also Berserk and go Kamikazi more.
Catch: 1.5x NPCs are more likely to throw grenades. NPCs always drop two grenades when dying.
Fog: 1.5x No motion tracker.
Famine: 2x Dropped weapons have half the initial ammo/charge.
Thunderstorm: 1.5x Maximum rank for all NPCs.
Tilt: 2x Damage type resistances/weaknesses are amplified.
Mythic: 2x NPC's health is doubled. (Mythic is also considered the difficulty level above Legendary.)
