As the previous answer stated, use a combination of Oomph (or Oomph Powder if you cannot cast Oomph) and Psyco/Psyche Up (or Timbrels of Tension if you don't have that) before you scout. The higher your attack power, the higher the scout chance. Also equip weapons that say they have an advantage over the type of foe you are scouting, for a slight boost. Don't bother lowering the monster's defense, as due to the scout damage formulas, this has no effect. Lowering the monster's HP to critical (red levels) does seem to give a boost to scout chances.

You should look at the synthesize list at Woodus and synthesize one or two rank A monsters.

www.woodus.com/den/games/dqm5ds/synthesizing.php

Keep the boxes about results that have the same rank, and rank decreases checked off so that you only get rank A results and better. Check off search in ingredients, and then select Skeleton Soldier or one of your other monsters in the drop down list, then search. It'll bring up a list of everything that your monster can be combined with, and what it will make, which is rank A and higher. Jum can be made into a rank A Mum, Boe, or Boh for example, which can all be combined together in a 4 way synthesis to form Mumboh-Jumboe. Skeleton Soldier can be combined with the free Drakularge you get once you have obtained 100 species of monsters at different times to make the rank A Great Dragon. An Abyss Diver and a Stark raven can form a rank A Heligator.

Focus on synthesizing new monsters, and then after you have some strong monsters start scouting rarer ones.