Here's a proposition: Iku Nagae, best MND tank in the game?
I'm being serious here. I spent 80 levels raising MAG-based Iku, and at the end, I realized that my Iku still had better MND than MND-based Minoriko. Curious, I went to check up on her MND growth, only to realize that her growth rate is 11! That's just 1 point lower than Tenshi's, and Iku has a better level up rate and HP growth! It's kind of disappointing that the wiki currently lists her survivability as "passable, but nothing special" when she may well be one of the best magic tanks in the game.
Compare Iku to Yuyuko and Patchy, who have the two highest MND growths in the game. Yes, Patchy's MND is nigh-insurmountable, but Iku has far better HP, and levels up way faster than either of the other two. The biggest boon for Iku, though, is that she doesn't need to put her levels into MAG. While Patchy and Yuyuko are only here to do damage, Iku doesn't need MAG to buff her allies or debuff her opponents. This frees up her level up bonuses for raising her already great MND, possibly turning her into an incredible magic tank. And, as a side note, Iku has a higher DEF growth than both Patchy and Yuyuko, and might beat Yuyuko in HP too due to her faster level up rate.
Tenshi might still be better overall due to being able to raise her own DEF/MND, but if you can buff Iku with Reimu or Minoriko or whoever, then you're looking at one fantastic MND tank. I regret spending all those levels on MAG now; MND-tank Iku is the way to go.