There are three ways, that I know of.

You can wait till you are stronger and get two Behemoth slimes, and synthesize them to get an easy King Slime..

And the hardest way, to scout it on Infant Isle, which is impossible for people with F ranks.

So, here's what I did. Of course, I had a bunch of A ranks already, so this might have helped earlier. All you have to do, is scout four slimes, 2 negative, and 2 positive. When they hit level 10, synth them to get two slime+1s, hopefully one will be + and the other -. If so, level them both to level 10, and synth them. And there you have it, a King Slime+2, Rank C. The King Slime is not only more powerful and has a lot of health at a higher level, but mine can withstand a Rank X monster now. :) Thanks to synthing it with more and more slimes.

Hope I helped.

POPO a.k.a Swampert_King456 out.

P.S POPO, is my scout's name.

This hint is for a Quick level up follow these directions exactly.

1.Have a Monster with metal slash

2.Have 10000 Gold

3.Go Down in the sewers on Domus Isle talk to the blue-haired girl next to the weapon vender.

4.Pay 10000 Gold for the metal ticket

5.Go to Palaish Isle Go up the stairs next to the scoutpost Go to the Man that does the Garden Challenge.

6.He will ask if you want to use the Metal Ticket say yes.
Note. Go in with one monster with Metal Slash and go nuts and you will gain levels real quick.{Hope you got a deep Pockets cause this costs but it is worth it}