OK, so I just completed all of the Hero (amiibo) Battles in Fates.
From easiest to hardest, I would rank them:
Lucina's is a cake walk and it feels like they didn't even try to make it that challenging.
Marth's set up of troops is initially intimidating. You're on one side of a bridge, he's on the other side, and there is a cluster of his troops on the bridge. But I had units really well built for the job (i.e Rinkah's damage sponging abs).
Ike's is the most complex layout and has you on your toes, and I honestly would have just about lost if my avatar didn't evade a particular attack.
Robin's is kinda brilliant and feels perfect for the character. It's incredibly simplistic. It only has 4 units including himself and one he is paired up with, and he's at the other edge of the stage. However, try to approach him at all and you will wind up getting caught under a Hexing Rod that his unit with a rod has. Unless you are severely overleveled, this is impossible to just charge through, and that's why it's the most difficult. Robin's battle requires you to figure out a perfect solution to his perfect defenses, like a true tactician. However, I divised a strategy that worked perfectly (though it's slightly RNG based). I simply gave Felicia Hexing Rod and Silence, and had her Silence Robin's magic caster unit. This would sometimes fail, but I had Azura at the ready to give her another try in that event. The Hexing Rod was used to soften up the guard on the bridge, for Ryoma to take down with an Armourslayer. Then my avatar finished off the unit with the rod. Robin then attacked my avatar, putting him at just a sliver of HP left. Then Ryoma one-shotted Robin. Then I had Felicia use another Hexing Rod on the remaining unit, leaving him at a low enough HP for my avatar to one shot. It was really fun and satisfying to figure this one out.