Square Enix announced the Great Dragon Free Distribution Campaign in the Dragon Quest Monster Battle Scanner arcade game. The campaign will be held from April 29, 2017 and during this time you can get the Great Dragon card for FREE.  The Great Dragon gives a powerfull attack to all enemies so it is a perfect weapon for all the warriors.
The ending date of the campaign is unknown so try to get yours as soon as possible.
Here you can find more details: http://blog.jp.square-enix.com/dqmbs/2017/04/post-147.html