MapleStory2 Boss Location Guide has the locations of all the new bosses introduced in the New Lv. 40 Patch. This Guide is taken from the Korean Forums and made easier to find/understand here.
Elite Monsters appear only certain fields, just as the field boss particular time. The biggest difference between the Elite Monsters and Elite Bosses is HP. A few hundred thousand (100,000) Elite Monsters are not less than 1 million. The dropped item rarity is capped at Elite Items. These Elite Monsters tend to drop just crystals and lower grade gearing. Repeatedly moving through channels to hunt Elite Monsters can be beneficial to obtain many Crystal.
Field Bosses appear only in certain fields at a specific time. All people in the channel are allowed to participate in the battle. There is No time limit to kill the boss. Rare Items also be obtained from the previous existing top rated bosses such as “Griffy Wings”. Many player farm these bosses looking for the many Excellent Items they can obtain. If you’re lucky, you may also get Premium Items drop such as Mounts.
Dungeon Bosses are recommended to be done in a Party of 10 People. These bosses feature a Time Limit and require Keys in order to attempt these dungeons. These bosses have high-levels of HP and also requires somewhat of a high degree of skill in order to defeat it. Dungeon Boss Monsters currently reward players with the best items that can be obtained. Also an easy way to obtain Chaos Crystals. Keys: You have 10/10 Keys that can be used to enter these Dungeons, they are used upon entry and regenerate after hour.