In addition to all of the other solutions, here are the Mastery milestones and descriptions:
Goliath*** Spoiler - click to reveal ***Rock Throw
1 - 12,000 damage
2 - Damage multiple targets, 35 times
3 - Hit mid-air Hunters, 45 times
Leap Smash
1 - 12,000 damage
2 - Damage multiple targets, 29 times
3 - Hit Hunters from at least 20 meters, 50 times
Fire Breath
1 - 12,000 damage
2 - Damage multiple targets, 110 times
3 - 8,500 damage to mid-air Hunters
Charge
1 - 8,000 damage
2 - Damage multiple targets, 28 times
3 - Hit multiple Hunters, 53 times
Kraken*** Spoiler - click to reveal ***Lightning Strike
1 - 20,000 damage
2 - Damage multiple targets, 40 times
3 - Damage multiple Hunters, 80 times
Banshee Mines
1 - 20,000 damage
2 - 60,000 damage to Hunters
3 - Damage multiple Hunters, 60 times
Aftershock
1 - 14,500 damage
2 - Damage multiple targets, 26 times
3 - Damage multiple Hunters, 60 times
Vortex
1 - 23,000 damage
2 - Damage multiple Hunters, 75 times
3 - Knock back 360 mid-air Hunters
Wraith*** Spoiler - click to reveal ***Warp Blast
1 - 14,000 damage
2 - Damage multiple targets, 57 times
3 - Damage multiple Hunters, 15 times
Abduction
1 - Abduct 16 targets
2 - Abduct 20 mid-air Hunters
3 - Abduct Hunters from a team, 225 times
Decoy
1 - 12,000 damage with Decoy
2 - 32,000 damage to Hunters with Decoy
3 - Stealth pounce while invisible, 30 times
Supernova
1 - 34,500 damage while using Supernova
2 - 121,000 damage to Hunters while using Supernova
3 - Deal damage with abilities while using Supernova