Farm slimes. Level up. Upgrade skills. Repeat.
You just became an adventurer, and you want to grow stronger fast. A mysterious merchant overhears you and mentions a hidden cave where slimes keep spawning. He is willing to show you the way to the cave and give you the necessary tools to help with your slime farming. In exchange, he wants the stones that the slimes will drop.
Attack slimes for EXP. Pick up their bodies for stones. Feed those bodies to a mimic for gold.
You get to level up fast. He gets the stones he wants.
The deal
Use your weapon to kill slimes for EXP
Loot slime bodies for their stones
Feed the slime bodies to a mimic (each slime digests on its own timer)
Spend EXP on skills to raise base stats
Turn stones in to the merchant to improve the slime pit
Spend gold to multiply those base stats