Here is a Dead Island Riptide guide that helps you find all 13 locations of the champion monsters in the game. After killing all 13 champion monsters you can unlock the following achievement (and trophy):
"Hunter-Seeker" (90 Gamerscore / Gold Trophy)
- Find and kill all named champion monsters located in dead zones. Bonus: Lots of XP and bonus weapons.
- These monsters are all hidden in small side areas, the "dead zones".
- The first six can be found in the Flooded Jungle and the remaining seven in Henderson.
- The entrances to these side areas are marked on the world map. After entering a dead zone you will get a side quest to kill the champion.
- The champion is marked on the radar with a purple skull symbol.
- Toughness: The champions are stronger versions of standard monster types. Make sure you have some good weapons in your inventory and use your special skills to kill them quickly.
- Drops: They give lots of XP and also drop some nice weapons.
Skip to the times below for each specific champion monster:
- James "Killer" Nisira - 0:05
- Jimmy "Deathtrap" O'Neil - 0:22
- Peter "The Undying" W. Spicer - 0:36
- Olivier "Deathbone" Wenge - 0:52
- Joe "Ogre" Somare (Killed during story mission) - 1:07
- Eduardo "Quickdeath" Petoia - 1:29
- Frankie "The Scourge" O'Hara - 1:44
- Metzger "Skullcrusher" Sloat - 2:00
- Geoffrey "Walking Carcass" Nape - 2:18
- Doctor "Scar" J.J. Evans - 2:33
- George "Body Eather" Serero - 2:51
- Joel "Blackheart" Chubalba - 3:06
- Ogio "Blindspot" Moke - 3:22
Thanks to PowerPyx for the guide.