Once the Arena opens, choose "Monster Massacre" from the list of options. At the start of the battle, use the Decoy Knack to lure all the slimes in your direction. Then, have someone cast a level 1 area damage spell (for example, Blizzard). It should kill all them before anyone else gets to attack them. You can get twelve items if you kill all of the slimes. The items all increase your stats; for example, you can get the Life Apple that raises strength, the Wisdom Apple that raises intelligence, and another item that raises dexterity. It costs 3,000 to enter each time.
Growlanser: Heritage Of War