Level 35-39: The Jr Grupins in Ossyria are a good choice as you should be able to 2 or 3 hit KO them with minimum MP usage per attack, especially since level 1 MP Eater recovers 14 MP(Maple Story Mesos). You could also go clawing up Boars, which might be more profitable. Also, Jr Wraiths in the sewers of Kerning City provide good experience because they are very weak to fire. They also give 5 more experience in Maplestory each than Jr Grupins. White Leatties and Dark Leatties and Jr Pepes are good for El Nath. Very Profitable.
Level(MapleStory powerleveling) 40-48 : Kill things you can 1HKO. Jr Grupins and Jr Wraiths are good initially, then eventually you can 1HKO Jr Pepes semi-consistently. Note: At level 45, you might want to consider going to train on Extra's A and B at Showa Street 3 as they are weak to fire and you might be able to two-hit KO them. They also have much better HP/EXP RATIO than Jr Yetis although you will not net much profits.