You can find the task skull on Adventurer's Island
Minotaurs (250 kills):
Minotaur, Minotaur Archer, Minotaur Mage
Rewards: 5 crystal coins & 1,000,000 experience points
Rotworms (250 kills):
Rotworm, Carrion Worm, White Pale, Rotworm Queen
Rewards: 5 crystal coins & 1,000,000 experience points
Coryms (250 kills):
Corym Charlatan, Corym Skirmisher, Corym Vanguard
Rewards: 10 crystal coins & 1,000,000 experience points
Cyclops (250 kills):
Cyclops, Cyclops Drone, Cyclops Smith
Rewards: 10 crystal coins & 1,000,000 experience points
Stonerefiner (500 kills):
Stonerefiner
Rewards: 15 crystal coins & 1,000,000 experience points
Krailos Steppe (1000 kills):
Clomp, Ogre Shaman, Ogre Brute, Ogre Savage
Rewards: 30 crystal coins, 3,000,000 experience points
Wyrms (1000 kills):
Wyrm, Elder Wyrm
Rewards: 30 crystal coins & 3,000,000 experience points
Edron Vampire Crypt (1000 kills):
Vampires, Vampire Brides, Vampire Viscounts, Bonebeast, Necromancer
Rewards: 30 crystal coins & 3,000,000 experience points
Edron Demon Crypt (1000 kills):
Askarak Demon, Shaburak Demon, Askarak Lord, Shaburak Lord, Askarak Prince, Shaburak Prince
Rewards: 30 crystal coins & 3,000,000 experience points
Cults & Heros (1000 kills):
Hero, Vile Grandmaster, Renegade Knight, Vicious Squire, Cult Believer, Cult Enforce, Cult Scholar
Rewards: 30 crystal coins & 3,000,000 experience points
Werecreatures (1000 kills):
Werewolf, Werebear, Wereboar, Werebadger, Werefox
Rewards: 30 crystal coins & 3,000,000 experience points
Glooth Bandits (1000 kills):
Glooth Bandit, Glooth Brigand
Rewards: 30 crystal coins & 3,000,000 experience points
Hive (1500 kills):
Kollos, Spidris, Crawler, Insectoid Worker, Swarmer, Waspoid, Spitter, Spidris Elite, Hive Overseer
Rewards: 50 crystal coins & 5,000,000 experience points
Deeplings (1500 kills):
Deepling Brawler, Deepling Elite, Deepling Guard, Deepling Master Librarian, Deepling Scout, Deepling Spellsinger, Deepling Tyrant, Deepling Warrior, Deepling Worker,
Rewards: 50 crystal coins & 5,000,000 experience points
Oramond West (1500 kills):
Glooth Blob, Rot Elemental, Blood Beast, Devourer, Quara Constrictor Scout, Quara Pincher Scout, Quara Predator Scout, Quara Hydromancer Scout
Rewards: 50 crystal coins & 5,000,000 experience points
Carnivoras (1500 kills):
Lumbering Carnivor, Spiky Carnivor, Menacing Carnivor
Rewards: 50 crystal coins, 1 titanium token & 3,000,000 experience points
Asuras (1500 kills):
Dawnfire Asura, Midnight Asura, Frost Flower Asura, Hellspawn
Rewards: 50 crystal coins, 1 titanium token & 3,000,000 experience points
Werehyaenas (1500 kills):
Werehyaena, Werehyaena Shaman
Rewards: 50 crystal coins, 1 titanium token & 3,000,000 experience points
Werelions (1500 kills):
Werelion, Werelioness, White Lion
Rewards: 50 crystal coins, 1 titanium token & 3,000,000 experience points
Deeper Banuta (1500 kills):
Hydra, Medusa, Serpent Spawn, Behemoth, Eternal Guardian, Souleater
Rewards: 50 crystal coins, 1 titanium token & 3,000,000 experience points
Draken & Lizards (1500 kills):
Draken Warmaster, Draken Spellweaver, Lizard Dragon Priest, Lizard Legionnaire, Lizard High Guard, Lizard Chosen, Ghastly Dragon, Draken Abomination, Draken Elite
Rewards: 50 crystal coins, 1 titanium token & 3,000,000 experience points
Oramond Raids (1500 kills):
Minotaur Invader, Worm Priestess, Minotaur Hunter, Mooh'Tah Warrior, Minotaur Amazon, Moohtant, Worker Golem, Glooth Golem, Rustheap Golem, Metal Gargoyle, War Golem, Blood Beast, Drillworm, Wyrm, Devourer, Glooth Anemone, Behemoth, Elder Wyrm
Rewards: 50 crystal coins, 1 titanium token & 3,000,000 experience points
Spectres (1500 kills):
Gazer Spectre, Ripper Spectre, Burster Spectre, Arachnophobica, Thanatursus
Rewards: 50 crystal coins, 2 titanium token & 4,000,000 experience points
Deathlings (1500 kills):
Deathling Scout, Deathling Spellsinger
Rewards: 50 crystal coins, 2 titanium token & 4,000,000 experience points
Roshamuul (1500 kills):
Silencer, Guzzlemaw, Frazzlemaw, Retching Horror, Choking Fear
Rewards: 50 crystal coins, 2 titanium token & 4,000,000 experience points
Werecrocodiles & Weretigers (1500 kills):
Werecrocodile, Feral Werecrocodile, Werepanther, Weretiger, Cunning Werepanther, White Weretiger
Rewards: 50 crystal coins, 2 titanium token & 4,000,000 experience points
Warzone 5 (1500 kills):
Tunnel Tyrant, Cave Devourer, High Voltage Elemental
Rewards: 50 crystal coins, 2 titanium token & 4,000,000 experience points
Catacombs (1500 kills):
Glooth Golem, Rustheap Golem, Worker Golem, War Golem, Minotaur Amazon, Execowtioner, Moohtant, Destroyer, Hellspawn, Grim Reaper, Dark Torturer, Demon, Hellhound, Juggernaut
Rewards: 50 crystal coins, 2 titanium token & 4,000,000 experience points
Falcons (1500 kills):
Falcon Knight, Falcon Paladin
Rewards: 50 crystal coins, 2 titanium token & 4,000,000 experience points
Cobras (1500 kills):
Cobra Scout, Cobra Assassin, Cobra Vizier
Rewards: 50 crystal coins, 2 titanium token & 4,000,000 experience points
Summercourt (1500 kills):
Crazed Summer Vanguard, Crazed Summer Rearguard, Insane Siren, Arachnophobica, Thanatursus
Rewards: 50 crystal coins, 2 titanium token & 4,000,000 experience points
Wintercourt (1500 kills):
Crazed Winter Vanguard, Crazed Winter Rearguard, Soul-Broken Harbinger, Arachnophobica, Thanatursus
Rewards: 50 crystal coins, 2 titanium token & 4,000,000 experience points
Deep Desert (2500 kills):
Skeleton Elite Warrior, Undead Elite Warrior
Rewards: 50 crystal coins, 4 titanium token, exalted core, 6,000,000 experience points
Bulltaurs (2500 kills):
Bulltaur Brute, Bulltaur Alchemist, Bulltaur Forgepriest
Rewards: 50 crystal coins, 4 titanium token, exalted core & 6,000,000 experience points
Ferumbrass Seals (2500 kills):
Gimeleech, Vexclaw, Demon, Hellflayer, Spectre, Dark Torturer, Juggernaut, Hellfire Fighter
Rewards: 50 crystal coins, 4 titanium token, exalted core & 6,000,000 experience points
Lost Souls (2500 kills):
Flimsy Lost Soul, Mean Lost Soul, Freakish Lost Soul
Rewards: 50 crystal coins, 4 titanium token, exalted core & 6,000,000 experience points
Nimmersatts Breeding Ground (2500 kills):
Dragolisk, Mega Dragon, Wardragon
Rewards: 50 crystal coins, 4 titanium token, exalted core & 6,000,000 experience points
Issavi Sewers (2500 kills):
Black Sphinx Acolyte, Burning Gladiator, Priesstess of the Wild Sun
Rewards: 50 crystal coins, 4 titanium token, exalted core & 6,000,000 experience points
Issavi Surface (2500 kills):
Young Goanna, Adult Goanna, Manticore, Lamassu, Feral Sphinx
Rewards: 50 crystal coins, 4 titanium token, exalted core & 8,000,000 experience points
Ingol (2500 kills):
Harpy, Carnivostrich, Liodile, Rhindeer, Crape Man, Boar Man
Rewards: 50 crystal coins, 4 titanium token, exalted core & 8,000,000 experience points
Dragons (4000 kills):
Dragon, Dragon Hatchling, Dragon Lord, Dragon Lord Hatchling, Frost Dragon, Frost Dragon Hatchling
Rewards: 30 crystal coins, dragon claw, 3,000,000 experience points
Secret Library (5000 kills):
Flying Book, Ink Blob, Guardian of Tales, Rage Squid, Burning Book, Biting Book, Knowledge Elemental, Energuardian of Tales, Brain Squid, Energetic Book, Animated Feather, Squid Warden, Icecold Book, Cursed Book
Rewards: 50 crystal coins, 8 titanium token, exalted core & 12,000,000 experience points
Podzilla (5000 kills):
Rootthing Amber Shaper, Rootthing Bug Tracker, Rootthing Nutshell, Quara Raider, Quara Plunderer, Quara Looter
Rewards: 50 crystal coins, 8 titanium token, exalted core & 12,000,000 experience points
Gnomprona (7500 kills):
Sulphider, Sulphur Spouter, Nighthunter, Stalking Stalk, Undertaker, Headpecker, Mantosaurus, Mercurial Menace, Noxious Ripptor, Shrieking Cry-Stal, Emerald Tortoise, Gore Horn, Gorerilla, Hulking Prehemoth, Sabretooth
Rewards: 50 crystal coins, 12 titanium token, 2 exalted core & 25,000,000 experience points
Soul War (7500 kills):
Brachiodemon, Infernal Demon, Infernal Phantom, Branchy Crawler, Mould Phantom, Rotten Golem, Bony Sea Devil, Capricious Phantom, Turbulent Elemental, Cloak of Terror, Vibrant Phantom, Courage Leech, Many Faces, Distorted Phantom, Mirror Image
Rewards: 50 crystal coins, 12 titanium token, 2 exalted core & 25,000,000 experience points
Rotten Blood (10000 kills):
Sopping Corpus, Bloated Man-Maggot, Mycobiontic Beetle, Oozing Corpus, Oozing Carcass, Sopping Carcass, Rotten Man-Maggot, Meandering Mushroom, Darklight Emitter, Darklight Construct, Wandering Pillar, Converter, Walking Pillar, Darklight Source, Darklight Striker, Darklight Matter
Rewards: 50 crystal coins, 16 titanium token, 5 exalted core & 50,000,000 experience points