For better efficient for this direction, three space style NPCs and high ATK weapons (such as Anomalos club) will be helped.
And you need to equip space mitama for weapon and demon's hand. Mitama you would like to get skills should be equiped at armor.
Also, some skills for increasing spc spell damage can be usefull.

And then, go multi mission 7-1 (first mission at phase 7) which has two medium bosses


this is just all about this direction
right after starting, go between two bosses.
And then use X spell (I don't know its name) and aritamafuri(spell on demons hand).
If you prepared those three things (NPC, Anomalos club +12 on ATK, spc spells on style and demons hand) and cast spells between two, it will take about a minute.
It'll take a little longer if you do not prepare all three, but it won't take more than 5minutes.
And also, there's some skills which need much more exp than other skill and in that case, just do few more times.

Eye of truth seen masks can only be seen with demon's eye.
They are on fields but they do not regenerated fast ( also, you cannot use save & load).
And also, there's possibilities of other items than eye of true seen mask at hidden shinnings.

But there's some missions which have hidden shinning.
One mission is first mission at multi phase 1.

And If you jump down from cliff, there's some scluptures infront of you.

If you go a little bit further, there's one sculpture and hidden shinning is infront of this sculpture.
It'll be seen like this


And this also has possibilities of other items, so it will take few trials to get eye of true seen mask.

I got these information from http://bbs.ruliweb.com/game/82225/board/read/9409234?
thanks to them