1. How does this card stand on its own (analyze the HP, attacks, Abilities, etc. of the card)?
This card on it's own is amazing in this format. The two top decks are Lighting and Dark, which Donphan hits double on both decks. One energy for 60 is incredibly fast and when you get him built up, Three energy for 90 doesn't seem like much, but when you can't OHKO Donphan, he can hit 180 before getting KO'd, but when you're hitting weakness on the two biggest decks there are, you're bombing 180 damage anyways and mostly only get hit once, doing 180 per turn.
He has one downside, weakness to water. Empoleon has started seeing some play which means trouble for Donphan.
2. What cards, if any, does this card combo with?
One energy for 60 and very hard to OHKO, just looking at this, one card comes to mind, Max Potion. Fully healed and running again with ease. Second. Rocky Helmet. This card is MAJORLY important, more than a staple if you will, this card attached to Donphan Prime, WINS GAMES. for the weakness side, Zekrom EX or Darkrai EX with Eviolite, if Donphan hits first, not a chance. Hit for 180 with Heavy Impact and now you leave them with a choice: Hit for damage and lose two prizes, or don't hit, and lose two prizes. Some people think that Donphan doesn't do good against Mewtwo EX, but think about it, with a Rocky Helmet you attack for 60, they you for 60-100 (god only knows with Mewtwo) and take 20, if you pull a plus power, you've put them into the same position as a Darkrai, if they attack, you've now 2HKO'd a Mewtwo EX. Even if you don't draw into a plus power, if you hit a Max Potion, you can 3HKO Mewtwo with only taking one hit. Another card that helps is Ruins of Alph. Makes Pokemon Tornadus EX and Shaymin EX so much easier to KO. instead of the 5-9HKO on Tornadus EX, it moves to 3-5, and with Rocky Helmet, it drops to 2-4, and that's just with Earthquake. With Heavy Impact, Tornadus doesn't stand a chance.
3. What role does/could this card have in the metagame?
This is my section, I run a deck called DMRH, Donphan/Mewtwo EX/Rocky Helmet. It's a troll deck sure, but against Tetrakion, it will always win, whether you use Donphan or Mewtwo as the main. Against Eel variants, Donphan's Exoskeleton and resistance to Lightning let him take a Strong Volt from Zekrom EX and still attack. And with Zekrom being weak, it's in deep trouble unless it attacks first. Everything else in that deck is weak to Donphan...except Tornadus EX. When your opponent drops this, be ready for some problems. You will need two Donphans that have one energy and a Rocky Helmet attached. Played right, you can win the prize trade, but if you want to risk it, play Ruins, but be aware of Zekrom EX from that point on. Against Darkrai/Tonradus. No problems, set up three Donphans and a Mewtwo, get Ruins of Alph out, and you'll win no problem.
4. Give it a rating (out of 10), and explain why you have given it that rating.
My rating for Donphan Prime is 10/10 for the Pokemon itself. With the Poke-Body and the speed of Earthquake, this card is DANGEROUS. I only give 9/10 for card art, if Donphan's mouth was open, it would make him more menacing.
MIND YOU, DONPHAN'S EARTHQUAKE IS GREAT, BUT HEAVY IMPACT SHOULD BE USED, IT WINS GAMES.