It shouldn't matter that its not in half the decks, like sofo is. This card is just dumb. Nobody should be able to win with 1 creature buffed 12 times. Removing the +atk component would easily solve this. It still gives the intended upgrade to immortals, without making it a spammable instant-win. Immortals don't need an upgrade anyway, they only have one possible counter, which this card takes away. Taking an OP card and making it more OP doesn't make any sense to me.
Why nobody should win with buffs?
There are DO, rush, SoV, poison, Anti-Vampire, spells, UG and much more ways to win, is there any reason to barring a mega-buffed creature from the list?
And by 12 times, do you meant that is an Arena issue? Arena deck are intended to be OP, hence there are handsome rewards.
Yes it is spammable, only after you put down an expensive creature, by the time a fire rush is doing 20 damage per turn and already done 30, it is no mean justify to say this is anywhere close to "instant-win."
And immortals do need helps.
Yes they are nontargetable, but they cost 6
and a card slot (which worths around another 2 cost) to deal 4 damage per turn, and still have their weakness (Lot-of shields before SoW, another 2 shields after SoW.)
6 immortals only deal 24 damages even there are no shields, all six in the deck, almost as low as poison, nobody can win with this tiny damage.
Those SoW do helps, monument do the same, while they are not quinted, monumented creatures can receive other buffs, Wismortals must stick with that 6 per deck SoW.
What if you get 3 towers, 1 immortal, 5 SoW as your first 9 cards of a once-a-year draws?
You are just doing 24 damage at 3rd turn, when Fire got their once-a-year draw, you'd already dead in the next turn...