It is nice that you got all of that from one card, and the tap ability can have it's moments, but overall this isn't a card you're ever going to be very excited about playing
You choose an artifact, instant, or sorcery card from it and exile that card
You will often not be able to kill the biggest creature they have out, but I think this will work enough of the time that its worth playing at least one copy of
Use them to build a deck or play limited matches with friends
Again, this isn't to say reactive answers, like generic removal, aren't good
In other words, as long as Regidrago has 3 energies attached (the ones you need to declare its attack), you can choose any attack from any dragon in your discard pile