From what I've seen, the moves were randomly chosen. Dialga was using Fire Blast vs my Umbreon the moment the raid started.
I can attest the AI for this raid doesn't seem very smart. I went in with solo Azumarill and Palkia just spammed Hydro Pumps on me. After I tera'd I saw a couple Fire Blasts and Thunders, but as you can see all Azumarill needed to do was only press Play Rough. I didn't even bring Belly Drum.
Metal Burst is the one that counters both physical and special at 1.5x damage but without decreased priority.If it has Metal Burst you just take mental note to run phisical attackers.