On Xelphatol and the Actual Summoner.

[Duelle]

a dungeon boss that outright summons Garuda briefly for an attack.

That guy is a tempered thrall of Garuda. And he's only able to do that summoning because of the shitload of crystals present in Xelphatol.

The mistake in that whole mess was having the boss summon Garuda in the first place. Of course, alternatives are sparse, since you could have that boss summon Garuda-egi along with Chirada-egi and Suprana-egi. The problem there, though, is that the method player characters use for summoning is something that is derived from Allagan data, which means that beast tribes are not going to have the needed info to summon egi in the first place (along with the fact that storywise your SMN is one of a handful in the world that know how to summon egi). In short, it's a mess.

And the prevailing excuses are still lore and "the models are too big". I'm not buying that anymore, SE. You wrote yourself into a corner, and I would love to hear you squirm out of the hole you made for yourself.

Personally, I don't see any outs. They had their opportunity with Heavensward (since the SMN questline could have focused on strengthening egis while further developing the relationship between the SMN and their pets), but that was squandered with Dreadwyrm Trance, the systems that came with it, and the aether limitations excuse used for why you can't summon Ramuh/Leviathan/Shiva-egi.

As I mentioned before, SE also changed the rules behind primals because they were bent on going down the "let's disprove all faiths" path of storytelling, which did nothing to help (as I said, Sanic the Hegehog as a primal is entirely possible thanks to how they wrote that part of the HW story; and that's not a good thing).