I am going with the thread creator's stipulation:
"Erza is tired of Mira fans so she decides to go slap some sense into and assert her dominance by beating Mirajane without using any armor nor weapon...
Restrictions:
Erza:can't use any armour nor weapon.
Mira:can only use wingless BSS
All: have to stay within 10m of the other "
This is base Erza then she has nothing to put down BSS Mira. She doesn't need to replicate Laxus' AP to eventually beat base Erza. Erza can't even user her basic swords hence my answer. If this is base Erza who is inferior to the Erza Kyria fought the first time (she had some swords) then she loses.
This was already debunked by the fact that Juvia said Gray gave it his all vs Mira and Cana teased him for struggling with Mira before he himself agreed that he struggled.
Had Gray held back, Juvia, Cana and Gray himself would have pointed this out as opposed to the opposite.
Base Erza doesn't have the luxury of tanking all of Mira's attacks and exhausting her. Furthermore Mira has fought Lucy for a while, Skullion for a long while and then Gray who she lost to extreme diff while heavily injured without running out of magic power. Odds are, she isn't running out against base Erza who can't even use her benizakura sword to channel all of her power into attacking.
Mashima pretty much told you guys how strong Mira was by wanking her as beyond Gajeel and among the strongest with Laxus and Jellal; as someone beyond Gray significantly; as someone who stalemated Skullion in her weakest form and it's still not clicking. The portrayal is clear and at the risk of starting a shitstorm and allowing people to shift the argument, it's showing us that Mira at full power would even give Erza and Laxus at full power a lot of trouble. Mashima will always buff the S rank mages in the end, regardless of how Gray, Gajeel and even Natsu progress. And yes that includes Mirajane.
I can agree for some points and disagree for others. What makes you think that Satan Soul is Mirajane's weakest form ? In this form, she grew strong enough to compete equally with Jacob, a Spriggan. I think, the Spriggan is stronger than Seilha. So Satan Form is certainly above all the souls she took over in Tartaros, unless the "Pluto Grim" form, which is a One Shot move like Lucy's Mix Star Dress.
For Grey, beating him in Base Mode is not a crazy feat. We already know that Mirajane is stronger than Base Grey, Gajeel or Natsu. Natsu (Base) + Lucy couldn't take down Jacob in combat before tricks. She took him down alone in a hard fight. As I haven't seen Grey's best form in this combat I can't assume he used it. Still you can go all out and spend your magic until being exaust in Base form. Or are you assuming Natsu gave his best against Gajeel ? It's same way of fighting (people who haven't seen this fight and come to the final point can see Natsu as exaust as Gajeel to nearly a mismatch : but in the show, I haven't seen any Dragon King's spell or Dragon Force to put him down). I also can assume that Elfman was still aside Mirajane and even exaust, can still support her. So Grey faced an exausted team up while the girls split to take Makarov, Gajeel and Reby in the cards the time they fought.
Yes, Mirajane is strong, agree, and Elfman as well as I can class him Natsu/Grey/Gajeel's base level. But no, I don't think Mirajane is on par or can give troubles to the like of Erza/Laxus/Jellal. In fact, I can place her in the middle Tier between the current S-Classes and Natsu-Grey-Gajeel-Elfman's level.
Erza's powerlevel is also base on her psychologic mood. If she is confident, Mirajane can't take her down. I think she is still too durable.
If she is doubtfull, perhaps she can take average under these conditions, but clearly not even sure.
For Kyria, she took Erza down only with hax (even the first time while Erza was with a normal sword), which hax is now useless against Erza's artificial's eye. Not sure Kyria could have taken down Erza without her hax as I think Erza is still too durable physically.
But we can agree to disagree, no problem with that.