You will only train at a reasonable pace if you consistently use your skills. Never use a "regular" attack for any class after level(Maplestory powerleveling) 25, and if you're an assassin or mage you will probably need to use skills way before that. To keep up the pace of your mana-consumption, you must buy potions to consistently up your mana. By level 35 mages should hold at least 60 mana potions so they don't have to go back and forth to buy more. However, smarter players save mp by using they regular attack on monsters that have very low health.
Additionally, players must always have HP(Maple Story Meso) potions ready. Sitting still for 10 minutes every time when hit is not a fast way to train. By level 35, any class should have 50 orange potions. Upon reaching level 40, start scrolling weapons, on top of using your skills, to keep a reasonable pace of experience. 60% scrolls are the general recommendation, and if feeling lucky, buy 10% scrolls off of other people. 100% are a safe choice, but add less than 60%.
For assassins, get the best stars you can afford at all times. Kumbi tend to be the ones people get the most. Or, if funded, get icicles at early levels and kill much faster. If you have MapleStory iTCG, and play in MapleGlobal, and are funded, you may want to try for Crystal Ilbi, which is +29attk.