After laddering to Gold in the AG tier, I have a few thoughts about the core (and possibly future core challenges).
While laddering through mid-ladder, the experience felt similar to a normal ladder experience, with the core not being particularly difficult to utilize. Waterceus + Ho-oh is already a fantastic core (one that I used extensively in Gen 6), and the core didn't seem to challenge the laddering experience much.
Confirming that Ho-oh is a fantastic teammate is perfectly fine when paired with a Supportceus, but I personally think that the cores in the Core Challenges can be a bit more creative and provide more of a challenge so the ladder requirements can be
lowered to a reasonable amount. (Platinum that's past #1 seems a bit too far fetched)
Some ideas for future AG cores:
Giratina-Origin and Magearna combined can check most of the threats in the metagame while maintaining a fantastic offensive presence. Both of these mons are heavily underused, so it would be interesting to see how people would attempt to use this unlikely duo.
Lando-T provides an offensive check to Primal-Groudon and Ho-oh, allowing Xerneas to attempt a sweep. While Xerneas is a domineering presence in the metagame, Landorus-Therian is almost extinct; this core thus showcases the multifaceted AG tier.
Overall, I think that while this AG core challenge is certainly a fantastic start, I'm hoping that in the future, unique and challenging cores can make their way into this challenge to help add variety to the metagame and challenge users to come up with creative ways to utilize the cores provided.