User blog comment:VeiRetning/What UR Cards are worthy for Mirror Maiden?/@comment-32987645-20170830045907/@comment-29974411-20170830075320

Hi there,

I would think it's not quite worth to use an MM on cards with 300% ATT/DEF like Phantom Ninja 300% although it is a pretty good upgrade for u currently.

The main reason for the above is that there have been events that gave out "Free" 250% ATT/DEF cards for FAW kills or Item Exchange, examples are

Summer Alchemist during A Celestial Vacation by exchanging [White-Wave Seashell] items

Cranky Relena during Holy Night! No One at the Gate from FAW Subjugation

Thus Phantom Ninja is not too big of an upgrade compared to what we could possibly get for "Free" by playing the game and therefore the MM you have is probably better to be used on a 400%+ All Element ATT Buffer or a 10x Multihitter.

That said, it isn't a total waste either(Just a lot less worthy) to MM a Phantom Ninja as we always need a number of buffers to fill up our many Adv/Adv+ Elemental Hall/Extreme Tower Teams in the long term. Also for the events I mentioned above, they are about half year apart, and the most recent "Free" buffer, Inspired Relena was limited to only players with Teams that can do Advance or Higher Elemental Hall, and the other time Inspired Relena returned was during an ABB and base on the amount of rings needed to get her, she is kind of limited to players who cash during that ABB. Also Phantom Ninja seems to be, although not big, but decent upgrade for you at the moment from your description.

Well, just putting everything here for your consideration. Ultimately, the decision is still up to you :)