Gajeel is a physical fighter who combats strong opponents with his physical abilities; the only person Brandish has ever fought hand-to-hand is Lucy. Brandish is the one who has to prove she's on par with Gajeel, not the other way. DS bypasses heavy resistance and without that Gajeel would have never phased Aldron, so trying to scale Brandish to that makes no sense. The Dragons blitz the Spriggan, and they scale to Animus who was a no named dragon yet was giving Natsu the business.
Yep.
And the funny thing is that the parent dragon's portrayal is >> Animus'
They were the dragons Igneel chose in his plans to confront Acnologia in the future. He wouldn't choose some scrubs that could be one shot by Acno, like Animus could. It is very likely that they were the strongest dragons of that time of their respective elements/sub-species, like how Igneel was the strongest fire dragon.
Plus. The parent dragons, to this day, has the best speed feat in the verse.
We now know Serena can eat the attacks of Weisslogia & Skiadrum along with Grandeeney's wind attacks. How does this change the outcome?
It doesn't change the outcome. It only give him immunity to their element. Serena gets blitz and one shot physically.
These were dragons that traversed across the entire continent within a minute, each destroying ~ 750 faces (total was ~ 3000).
Skiadrum (IIRC) was so fast that he appeared from outside the frame of view of the flying Pegasus, one shotted a Face, and disappeared from the frame of view of the Pegasus.....all within the span of a second.