Here is the full list of all 41 Cubers: Arena achievements worth 1,000 gamerscore.
Kill Cubitaur
Kill Garathus
Kill Thar
Kill the King
Let 10 enemies die from Rock Monster's stone in Cave arena
Kill 20 Necromancer's skeletons during a single wave in Cave arena
Kill dynamite tosser with his own dynamite in Colosseum arena.
Let a boar kill 15 enemies before killing it in Colosseum arena
Kill 20 enemies while blinded in Abyss arena
Stand in a windmill trap for 30 seconds in Abyss arena
Freeze 7 enemies at once with a Crystal Trap in Village arena
Stun 15 enemies with a deflected Electric Worm projectile in a single wave in Village arena
Earn Max Rank
Complete any arena stage with two players
Upgrade an item
Socket a gem
Kill an enemy with a deflected projectile
Collect Healing Pickup while having 5% health or less
Collect 8 gold bags within 5 seconds
Perform a combo of 20 kills
Survive initiation arena in Oasis
Complete any arena without losing any health
Win Versus game on any difficulty
Buy new piece of gear
Finish arena wave while having 5% health or less
Equip a legendary item
Let Cubitaur eat meat 5 times, and then defeat him
Defeat Thar while he is empowered by lightning
Don't get hit by any electric bolts during the fight with Thar
Don't destroy any eggs during the fight with Garathus
Let Garathus eat 5 baby worms while he's hiding underground
Don't let King heal from Healing Slimes
Don't get hit by any tornado during King boss fight
Complete Time Attack on any difficulty
Complete Wave Survival on any difficulty
Complete Defender on any difficulty
Complete Mushrooming on any difficulty
Win Dodgeball game on any difficulty
Win Sparring game on any difficulty
Win Whackamole game on any difficulty
Win Wrestling game on any difficulty