Just gonna say now that I ramble a lot in this post about the idea of Tara-Season 7 combo.
I think that if Tara had lived past "Seeing Red", Willow's 'arc', if it can even be called that, in Season 7 would have gone a lot differently. Willow probably still would have been very afraid to use Magick after the Dark Willow stage at the end of Season 6. Tara would have been trying to help her out through that while trying to figure out where she and her are going to go from what happened.
Tara, in her own right, could have had an amazing arc in Season 7, IMO. I see alot of people claim/say that they didn't like her until towards the end of S6. If she had survived the gunshot wound from "SR", then it quite possible she would have discovered more and more who she was, and eventually just come into her own as a person. I could see the First taunting her with her Mother in "Conversations With Dead People", since that scenario would have been upsetting to her, much like how Dawn is affected in the episode. Not to mention the fact that it could have been interesting to see her interact with some of the Potentials. I could see her acting like a motherly figure to some of them, since she was always kind to everyone. Not to mention that in the episode where the Potential is actually the First in disguise (I forget the episode and the Potential's name, forgive me), she could have actually seen through the First's disguise maybe, considering her ability to see people's auras.
In "Chosen", she herself could have tried to reassure Willow about doing the spell to activate the Potentials. With her and Willow's combined abilties, the Spell might have been more powerful than it was with just Willow doing it. It's quite possible that since Willow would have been doing the majority of the spell-work necessary with the entire Scythe-Magick-Potential combo, she would have been the one to help Willow back to the bus after the spell. She'd, of course, be slightly drained from exerting her Magick in the way that Willow did, but the idea that she and Willow could use Magick on such a scale might have given her reasoning to see that, if guided and with the right motivations, Willow could use Magick on a larger scale. She more than likely wouldn't have done it herself in the same way, but still...
Not sure what you mean by preventing the majority of the battle. The battle was bound to happen. I mean, how else could they have stopped the First?