One way to use Belch would be to use a berry that restores health. I recommend the pinch berries over Sitrus and Oran berries because they restore more health.
Basically, attack your target until you consume your berry. As soon as you eat your berry, you can use Belch over and over again. This is a great move because it counters the growing number of Fairy Types in competitive. Your Hydreigon is extremely fast, so it will probably go before most Fairy Types.
Also try to replace Assurance with Dark Pulse. Hydreigon's higher Sp. Atk, Timid Nature, and Nasty Plot means Dark Pulse will do more damage overall.
Try this technique, and hopefully your Hydreigon will be a nightmare for Roosting Togekisses and Calm Minding Sylveons.
Hope this helps :D!