Easy way to get negative/positive monsters when synthesizing.
In the battle arena there is a room where if you helped the mud mannequin in Treepidation get it's heart you can do what I consider side quests. These side quests are called the Scout Proficiency Tests. Doing these quests give you several items. Two of these are the Plus and Minus sceptres. Give a monster you plan to synthesis the sceptre with what you want the offspring to be. Doing that should grantee that the monster you synthesis will always have the + or - depending on which one you were going for.
Added by: Ninetailed Fox Nov 13th 2011, ID#16390