According to
Thief Training Reference - MapleWiki.
Assassin Leveling AreasExp Areas40~47 Garden of Green / Red / Yellow (Jr. Cellion / Jr. Lioner / Jr. Grupin)
40~47 Cloud Terrace <4> (Teddy / Pink Teddy)
40~50 Subway Line 1 Area 2 (Jr. Wraith)
40~50 Cloud Terrace <5> (Toy Trojan)
50~70 Forest of Dead Trees I-IV (Coolie Zombie)
65~70 Sanctuary Entrance I (Cold Eye / Tauromacis)
65~70 Icy Cold Field (Yeti)
65~70 Forgotten Path Of Time<1> (Soul Teddy / Master Soul Teddy)
Money Areas40~50 Garden of Green / Red / Yellow (Jr. Cellion / Jr. Lioner / Jr. Grupin)
50~60 The Path of Time <1> (Platoon Chronos)
50~60 Mateon Field (Mateon)
50~60 Watch Out for Icy Path I (Jr. Yeti)
50~60 Cloud Park VI (Luster Pixie)
50~60 Cold Field I (Hector)
50~60 Golem's Temple IV (Mixed Golem)
60~70 Forest of Dead Trees I-IV (Coolie Zombie)
60~70 Dangerous Valley II (Red Drake)
All Around Areas30~40 Crow Forest / Crow Forest 2
IF your RICH(Crow)
30~40 Wild Boar Land I / Wild Boar Land II (Wild Boar) monster carnival is awsome training 40~50 Purple Plains 1 (Sakura Cellion)
GOOD MESOS DROP monster carnaval is awsome training sadly its lvl 30-50 only
40~50 Forest of Animals (Cloud Fox / Flaming Raccoon)
40~50 The Mountain of Cloud Fox (Cloud Fox)
IF your RICH50~60 The Dangerous Cave (Flyeye)
50~70 The Forest Of Gollem / Gollem's Temple IV (Mixed Golem)
60~70 Cold Field I (Hector)