Location:
Mines, Floors 1-30. The first 'boss' encounter is often considered the large Green Slime on Floor 5, which drops the Adventurer's Guild membership.
Strategy:
Green Slimes are basic enemies. The larger ones have more health but follow the same attack pattern: they jump towards you. Use your starting sword and swing repeatedly. If you get hit, eat some foraged items (like Spring Onions) to restore health. There's no special weakness; just keep attacking. The large slime on Floor 5 is a gatekeeper for the Adventurer's Guild.
Rewards:
Slime, occasional rings, and access to the Adventurer's Guild (after defeating 10 Slimes).