Seeing as Gray has repeatedly used his ice devil slayer magic in this 100 year quest, and even in this arc against opponents that Mirajane stalemated, your assumption that he didn’t have to use it to beat her makes absolutely no sense.
If the series does it, it’s not downplay, it’s just fact.
Elfman is not just as strong as Mira, him being just as strong as her as pointed out above would make no sense storyline wise and that’s not counting the fact that Macro can be used to empower people or the fact that Mirajane had a friendship boosted Elfman sneak attack a drained Seilah.
I mean, the joke was that he absolutely struggled with the fight and was on the verge of passing out afterwards and literally blamed only Mirajane for being the reason he was the way he was.