Big Toad in Toad Valley for the event quest chain, quest nr. 5
For this quest you got to kill Big Toad in Toad Valley 20 times.
First of all you can enter the dungeon 15 times / day so it'll be 2 days to complete it.
Now as I've seen people(me included) having problems understanding how to kill the Toad as he takes no damage from skills, I'll write a short "guide" so to speak you people don't have to strugle or ask around.
First you need to go to Toad Town, there you will have to talk with the Chief(old beardy guy in the middle), take the first quest he offers which will require you to obtain 3 secret pieces. For this you'll have to do the other 3 dungeons that are around Toad Town, first is Toad Forest(dungeon), then Toad Mountain(dungeon) and then Toad Cave(dungeon) respectively. At the end of each of the 3 dungeons enumerated you'll find a box, destroy it and you'll get the secret for the quest.
When the quest is complete, head back to Toad Town and hand in the quest at the chief. This will give you the skill "Salt sprinkle". You can find it in you skill window under support skills, it's a green skill.
To kill the Big Toad in Toad Valley(dungoen), you'll have to use this skill on him until he dies, then use regular skills to kill the small frogs until he spawns again, then rinse and repeat for 4-5 times until you get the kill and the loot.
Hope this helps