I go by the reasoning of we fight solo unless stated otherwise, due to the absurd mental hoops you have to go through to try and fit other adventurers into it.
Ifrit: everyone is tied up and taken where the hell the other 3 echo users come from, hm/EX can be fine either way.
Titan: Y'shtola barely was able to send us alone and had to wait until she can strengthen it for herself to use
Garuda: we landed with just Cid and Alphinaud, how do 3 other people get through her storm barrier to joins us and disappear mid way fighting her.
Utima: actually stated to have help
Mog: either way can happen due to how to get there
Levi: again either is possible
Ramuh: challenged us on the spot with only scions around, 7 adventurers getting through the slyphlands at that time, yea no.
Shiva: Nope just solo, Ysayle never ever mentions you beat her up with people on top of how we got there I doubt 7 heretics decided to join us to take down their saint.
Nabriales: Nope no time to gather 7 people that would have to all conveniently die before we leave.
Vishap: actually stated to have help
Coils: 4 people knew about it until after it was dealt with and even then only city leaders have the knowledge outside Scions (though Ruby Weapon has just blurred this a bit but until details about it can't say anything)
That is just ARR, let alone Warring Triad/Thordan/Ravana/Nidhogg of HW, 4 lords/Lakshmi/Susano (which they actually joke about)/Tsukiyomi of SB, Titania/Innocence/Hades of ShB.
It is a lot of hoops to try and justify canonically we had 3-7 people there to help us so I tend to go story wise we do it solo but gameplay wise we do as a group.
