Once you complete the campaign you will unlock King difficulty for each area. After progressing through the quests for beating each stage on King difficulty you will unlock a new dungeon called Floating Gardens in the Lorian area to the right of the map.
As the achievement states this must be completed under 45 minutes in online co-op. This is essentially survival set to Medium difficulty but still challenging enemies, there are 50 floors containing most enemies seen throughout the game. However the part that makes this frustrating is potions are disabled therefore healing is restricted however fortune coins can be use *see Edit below for further information*, due to this resurrection tokens are required.
***NOTE: Update:6/6/16 from MattiasAnderson.
Additional resurection tokens can no longer be purchased so it is advised saving most of the tokens you get at the start of the game for this challenge. If you have used all your tokens prior to obtaining this achievement your best bet will be finding a willing host who has their tokens available then doing the exit rejoin tactic with a full party.***
I used around 15 in a failed run with 2 players. I would recommend having atleast 40 tokens available before attempting this level and also using your best items. Due to the tokens costing MSP it is understandable people may not wish to purchase them, if this is the case then you can search for public lobbies and upon death exit the lobby to main menu, simply rejoin the host and you get full HP and Mana without using a token. This is the way I did it and with 3 players dropping in and out the host used around 18 tokens and with a time of 40 minutes. However using this is not a 100% tactic as the host may run out of tokens or the lobby may become full, but it is free.
The more players taking part the easier this is due to the extra damage available, enabling areas to be cleared faster saving both time and precious HP.
Also a note: Due to the number of glitches this game likes to throw at players I would recommend carrying thunder grenades as sometimes enemies can get glitched and are unkillable, costing valuable time. Simply throw a thunder grenade at the enemy to knock them out of their glitched state to be killed.
Edit: You must be an alive, active player upon completion to gain credit for beating the area. Thanks to xDartz xNVx
Edit 2: It is advised players speak to the monk in town to activate the quest before attempting to complete floating gardens or you may have to retry this. Thanks to Metamania2313
It also appears fortune coins can be used as healing items, I have never used these but upon looking into this I found a tactic posted on another site. "Also the key is using those fortune coins right.
If you use it and you hear a lightning (miss) sound, don't watch the HP going full down by 50%, but immediately use it again till you hear the sound of heal." you do not have to worry about dying from using them because they will never kill you. They will only take you as low as 1 HP. Credit to PlaneswalkerX from x360a. Thanks to Metamania2313 and xDartz xNVx for bringing this to my attention.
I hope this helps and add some insight into the challenge before you attempt it. Good luck.
Any amendments or comments leave them below, and I will review them.