I got the implication that the reason why she decided to Dynamax Togekiss is because Sirfetch'd dealt a significant blow to her Garchomp and she wanted to make sure the ace she used her gimmick with was at full capacity. Remember Meteor Assault absolutely HURTS just in terms of raw damage and Garchomp clearly took a good brunt of it. After all, if she were to just Mega Evolve an injured Garchomp against a Mega Lucario there's a risk that it could get taken out quickly and then leave Togekiss (who is weak to Lucario's steel typing) at greater risk.The episode was pretty good, obviously with the underdeveloped JN team and animation being solid especially for JN but not reaching the heights of XY, something about a certain fire-haired monkey being missing, it wasn't perfect and perhaps underwhelming for a battle that fans wanted for over a decade. Definitely wasn't expecting Dynamax Togekiss and I feel neutral about the decision, see what it does next week. Garchomp having 3 Dragon type attacks is questionable, but I can give them the benefit of the doubt as Scale Shot showcases an IoA move and resembles Stone Edge's old animation in the anime prior to the change in XY. The sneaky pebbles were a cool addition to Garchomp and could be used next week as its PLA version or how it would usually appear in the anime as an attack. And of course, the duck and fish got their redemption after their questionable Drasna battle and doing poorly in most other battles like Bea and Steven, which was heartwarming to see. I liked how Garchomp clearly faltered after that direct hit with the Meteor Assault - one gripe I had with the otherwise amazing Ash vs Kukui battle was that everyone made such a huge deal about Lycanroc hitting Incineroar with Stone Edge shards, when all the damage seemed to be a little pain done on Incineroar's arm. Anyways, Lucario should have a good chance to shine next week with some damage tacked on to Garchomp, and an entire episode next week should be enough time to prevent a situation like the Raihan battle where we were cheated out of an extended battle with Duraludon, as in the Pokémon who was showcased as being pretty equal to even Leon's Charizard but ended up getting G-Max Aura Sphere'd out of existence.
Dynamaxing Togekiss to lay on some serious damage on Lucario first (especially since Togekiss DOES pack moves that can leave a decent amount of hurt on it like Aura Sphere), and then bringing her ace back in to finish the job if Togekiss goes down sounds pretty solid to me.
On a side note: I'm waiting for the twist that her Togekiss is scarfed considering the air slashes she's spamming, lol.