Mewgenics

To complete the Caves with a Collarless cat, the cat must be in your party from the start of the adventure. Completing the Caves with a Collarless cat unlocks the Metronome active ability, which casts a random spell from the game's ability pool. Picking up a Collarless cat mid-run will not work for this unlock.

Source: Mewgenics Wiki

To beat the Boneyard with the Thief, utilize the class's agility and movement skills to evade undead creatures. The Thief's kit allows for speedy attacks and backstabbing, which can be effective against the skeletons, zombies, and spirits in the Boneyard. Additionally, breaking destructible Gravestones can trigger risk-reward outcomes, such as dropping coins or summoning enemies.

Source: Mewgenics Wiki

The provided sources do not specifically mention the Mage, but to unlock areas in Mewgenics, complete the previous area for the first time. For example, complete The Alley to unlock The Sewers, and complete The Sewers to unlock The Caves.

Source: Mewgenics Wiki

To complete The Core with the Tank, utilize the Tank's knockback-focused toolkit to control enemy positioning and keep pressure off your party. The Tank's basic attack can be used as a knockback tool, and its abilities can redirect enemy aggression toward itself. Additionally, The Core area features pools of lava and hellish demons, so be prepared to face these challenges.

Source: Mewgenics Wiki

To beat the Moon level with the Cleric, utilize the Cleric's healing abilities to counter the alien enemies' psychic and gravity-based attacks. The low gravity environment affects movement and projectile trajectories, so adjust your strategy accordingly. The Cleric's resurrection ability can also be crucial in this level.

Source: Mewgenics Wiki