If you get the genders wrong for synthing, here is a good tip. Add a Plus Scepter to one monster of a certain set, then add a Minus Scepter to the other monster from the other set. A Darkonium Slime is a good relief if you are fighting strong monster, as its metal body trait. It also has good skills, as the Metal King Slime has the ability Slimer(pathetic!). It looks like a Gem Slime, but is red.
AKA Red Gem Slime