Dead Island Riptide

This will unlock after the Prologue, 'The Storm' main quest

You will unlock this during Chapter 2, 'New Beginnings' main quest

You will unlock this during Chapter 5, 'Stalwart Defense' man quest.

You will unlock this during Chapter 8, 'Front Row'. Simply clear out the cinema of all zombies inside

You will unlock this during Chapter 7, 'Terminal Siege' main quest.

You will unlock this during Chapter 9, 'Militarized Zone'.

You will unlock this during Chapter 10, 'Evacuation' main quest.

This will unlock during Chapter 13. Simply defeat Harlow

You will unlock this during Chapter 11, 'The Crash' main quest.

You will unlock this during Chapter 12, 'With The Tide' main quest

Simply play the game and you should get this achievement. You start Dead Island Riptide on level 15 so you should be able to reach level 50 in your first playthrough of the game.

This unlocks once you have defeated Harlow and make it onto the boat at the end of the game.

Dr. Kessler is located inside the Research Laboratory, in the Flooded Jungle. Dr. Kessler will give you one side quest called 'History Lesson'. Simply complete it and turn it in.

Cecil is located inside the Makeshift Hospital, in Henderson. Cecil will give you two side quests, 'The Darkness' & 'Healthcare'. Simply complete them and turn it in.
 

Obtainable in your second playthrough of the game with the same character. Simply play through the game and it should unlock.