You've been catering to this guy constantly if you've been playing through all of Ni no Kuni's side quests, and now he wants you to pitch in on one final Errand. Humour him for some good rewards.
There's a man in front of the fountain in Ding Dong Dell who appears to have something on his mind.
Oh, Derwin. Not content with everything you've already brought him, Derwin has taken up residence by the fountain in Ding Dong Dell, and his demands are more outrageous than ever. He wants some Fierce Creatures, and you'll have to defeat one before he'll reveal the next.
The Grubby Fug
The Grubby Fug is on Jack Frost's Playground, the island southeast of Yule. If you've completed previous bounty hunts, you'll know the Grubby Fug and its ilk quite well - and you'll probably also know that they're fairly pathetic combatants. Most regular battles are harder than this poor guy.
Surprisingly, Derwin will appear beside you after the battle, and he'll have the next beast lined up.
The Auric Collum
Your second battle is against another familiar beast, a potent member of the Collum family. Travel to Al Mamoon and fly to the Genie's Steps in the south; the Auric Collum will be on a rise southwest of the Vault of Tears. It's a physical bruiser, but wind and electrical magic can chip it down to size. Watch out for Wallop and you'll get through this just fine.
Derwin again appears. Talk to him to get your last assignment.
This rowdy beast awaits on No Longer Mine, an island off the southern coast of Autumnia. The Shellmet is another of those blasted turtles, and it's even easier than its brethren. It will slowly follow whomever you're controlling and try to slap them with its shell. Keep Oliver at a fair distance and blast it with spells. If you're careful you may be able to weather this fight without taking a single hit.
Derwin has mysteriously followed you again, and when the battle's over you can talk to him to receive your reward: a Lumberwood Ticket, a Relixx Ticket and a Dinoceros Ticket, as well as 8000 guilders and eight Merit Stamps. The Tickets can be presented to Supreme Sage Solomon in the Temple of Trials in exchange for rare familiars. The Dinoceros one in particular can break your game, as it's fantastically strong.