Hi
The best way to succeed is to take care of it after finishing the story and a maximum of mission to be able to do the mission in expert mode "experience" which gives 25 000 experience points.
I did it for the first time at level 66 with attack G.
Take your character with 2 other people and climb the 70 and take another two, etc.
(the success is shorter than you think I do not know how, but by doing this quest thoroughly I noticed at one point that all the characters increased levels)
That is to say at one point I knew that my characters were at levels 60 except 2 and after increasing some I saw that they were almost all 66 my last 2 were at levels 68 when I started the quests with them lol