Training Area Level(Maple Story Mesos) 101 - 110:
Level 101 - 110 Grim Phantom Watches in Ludibrium. Because the thing about this map is that it's very large, the Grim Phantom watches are spread out on multiple platforms, this makes it very time consuming for a White Knight to walk around and kill Grims. But, nonetheless, Grim Phantom Watches provide good EXP.
Training Area Level 111 - 120:
From Level 111 - 120 Blue Wyverns in Leafre. The map is small and there is a relatively fast spawn for Wyverns here. Although pot costly, Blue Wyverns make the best solo training choice for White Knights around this level(Maplestory powerleveling). A bad thing about the map, if you do you will be sent to the previous map though is that there is a pit you can fall into at the bottom of the map.
Training Area Level 121+:
Training from this point really depends on the skills in Maplestory that you have obtained in your 4th job. If you have Divine or Holy Charge, when you are done waiting those 4 levels you can go and solo himes or you can train with a priest, wait for about 4 levels on it while putting skill points on it. Depending on your strength, you can also try to fight skelesaurus or skelegons too using your Divine or Holy Charge.