17 Nov 2017
The two abilities aren’t mutually exclusive. If a Dragon with flying (which is most of them) enters the battlefield under your control, both abilities will trigger.
17 Nov 2017
If a Dragon entering the battlefield causes Dragon Tempest’s second ability to trigger but leaves the battlefield before that ability resolves, that Dragon still deals damage.
17 Nov 2017
The amount of damage dealt by the Dragon that entered the battlefield is based on the number of Dragons you control when the ability resolves.