Way for me to cheat the system here! XD
I got Dragon Quest Monster Joker 2 today.
And even before my first arena battle I'll have 2 C class monsters!
(So I'll go from F to having 2 C class monsters in a few synthesis!)
Well I spent about 3 or so hours catching a training my draky's (while doing the storyline)
So by now I had 4 draky's
(2 +, 2 -)
Then I synthesised them into 2 drakys +1
(Both of opposite polarity)
By combining those I get....
Giant Draky
A class C monster!
(Its a great starting monster to get, as all Dragon Quest Monster Veterans know, they level up extremely quicky, they are very fast and strong, and make great starting synthesis material)
Then I scouted 2 slimes of opposite polarity, and am currently trying to get them to level 20.
2 Level 20 Slimes synthesised together makes a Slime X, which is also a class C Monster!

(Which I could then create another Slime X later and combine to make: Slime XY!)