You seem to be only considering the card in mono- 
Most of my counter-strategies were likewise isolated cards. But the person I was responding to was considering the card in a vacuum, so I responded in kind.
I can name other mixes that would be equally deadly to just about all of the strategies listed.
Please do. I will take anything you have to add along these lines into consideration as a possible reason to rebalance this card. It's honestly nice to have someone saying something more than "OP 4 immortal attack is hax."
Or flying weapons for that matter.
No, Mirror is not triggered by playing weapons or flying them.
Also, I don't think steal would actually be terribly effective; if a deck is counting on the mirrors for creatures then it probably doesn't have that many creature cards. So steal ends up as a more expensive version of explosion.
Running a deck that counts entirely on mirrors for creatures would be unwise - a single enemy creature played before your first mirror is enough to defeat you. Lots and lots of decks can pull that off. Steal is still a disincentive turned against its owner; even if the mirror deck has no creatures, the Steal-er doesn't
know that. The most valuable commodity in war is information - knowing any creature your opponent plays will be mirrored gives you a little breathing room in strategy.
To answer your question -
Aflatoxin does not trigger Mirror. It would be a viable strat used on either side - if the Mirror deck is using any targetable creature, a single Aflatoxin + Skull Shield will prevent Mirrors being triggered once the enemy field is full. If used on your side on a cheap early creature, creates a 23-attack army that will never create any enemy Phantoms, and will renew itself barring a Firestorm/Fire Buckler/Thorn Carapace.
Mindgate was a suggestion for
vs
. Used often enough, you can intercept cards and possibly gain some advantage. It's just a factor to consider in an even matchup - there are too many Mindgate possibilities for me to go into that.
As for Empathy decks, one of my favorites fits the bill nicely: FFQ. You get the first one out, then all the rest of the creatures trigger no Phantoms. Even if the first queen falls, tossing out a second shouldn't break you.
On a more general point, usually any time a card requires specific strategies against it that card is a little too strong.
Some ingame cards - or, I should say, deck concepts - already require specific strats to counter:
Quicksand Denial - must have a means of rapid quanta generation or at least two stacks of pillars/pendulums.
Flying Glories - must be outrushed or countered with insane healing.
Shrieker Rush - must be outrushed or have strong creature control, possibly also healing.
Poison Rush - must be outrushed, countered with plenty of healing, or deck must have Purify.
Unstable Gas - must be reflected or outhealed.
Growth decks, esp. Golems - must be outrushed or have strong early or non-damage creature control.
Sexington's - must have a large deck and/or Eternity with Enchant Artifact, plus a Photon or similar cheap creature.
Dissipation Shield/Field - must outrush, Quicksand, or have permanent control - only outrush if Enchant Artifact is also present.
Dune Scorpion decks - must be able to play many cards early and rely on a solid setup right away, or pack Purify. (unsure about this one)
A card is not over/underpowered based on isolated counters, but whether the presence of the card completely alters the game balance so that not including the card in any deck puts you at a serious disadvantage, and including it forces nearly any opponent to rethink his deck strategy. Of course as you point out, it can't be discussed in a vacuum - your "Awesome Shield" would be most deadly in a deck with Enchant Artifact, and that combo would absolutely break any creature-based deck, forcing massive restructuring of all decks that rely on creatures for damage.
To show any card OP, there needs to be a specific strategy (or strategies) that would render vast numbers of currently viable decks obsolete. It's like if firebolt rush could outpace any other deck known to man, and the only viable counter was a shield with spell reflection - everyone would have to run a firebolt deck with spell reflection, or they would fall victim repeatedly to those that do.
If it becomes clear that
or some other combination would be so powerful that everyone and their grandmothers would have to adapt to mass-migration or die, then a serious rebalance is in order, and I'll be very willing.
