Folks, let me tell you about Shadow Slime. Shadow Slime is a C rank Slime monster that learns Black Breath from its innate talent tree (Blighter). This is a dark-element breath attack that deals ~200 damage to everything at neutral resistance, and it can nearly single-handedly win every fight in D-tier tournaments and below. Now, what makes this monster special is how easy and quick it is to synthesize.
As soon as you get to Circle of Fortitude - Middle Echelon, you can scout a Beleth. You can then take a Beleth and fuse it with any rank Undead to get the D rank Demon Hypothermion. You then take that monster and fuse it with a regular Slime to get a Shadow Slime. So, you're jumping immediately from the E ranks to a C rank. But wait, there's more!
You can also fuse Shadow Slime with another easy to fuse monster, the Meowgician (E rank), to make a B rank Mogus (palette swap of Meowgician). All of its traits are elemental spell buffs, and it's going to get several spell talents just making it, so it makes an excellent spell caster. So if you wanted to, you can repeat the Shadow Slime process and then fuse a Meowgician by scouting a Vampire Cat (Circle of Temper - Lower Echelon story dungeon), and now you have both of these pretty strong monsters starting from just E and F ranks that you can scout normally.