1. How does this card stand on its own (analyze the HP, attacks, Abilities, etc. of the card)?
This card on it's own isn't good for a Stage 2 simply because of the Energy and effect of it's attack, I mean, it take four turn to manually charge, and you have to discard two of them when you attack, meaning it's another two turns til you can attack again.:l The ability is amazing, the dragon Klinklang for dark energy, sweet.

Retreat is pretty bad, and the weakness worries me because of Garchomp, Rayqauza and Giratina.
2. What cards, if any, does this card combo with?
Darkrai EX of course. It gives the heavy Dragon free retreat when it has a Dark Energy attached. And with Dark Patch, you can set him up faster by moving the energy onto Hydregion, the Psychic slot can be filled by Blend Energy GFPD, which can also be moved around thanks to the Dark aspect of it. Max Potion also works amazingly since you can move all the energy off, heal, then move it back on.
3. What role does/could this card have in the metagame?
Darkrai EX/Hydregion, or as I like to call it, The Dark-Region. By using Hydregion with Darkrai, you have the ability to change energy around, Max Potion, drop a card to the bench, built it in one turn, free retreat to send it active and attack. Most lists use Shaymin EX, but in my list, I use Giratina EX, against the Dragons, and the EX killer, Sigilyph. If I have four energy on the field, two of them Blend GFPD, I can move one Dark Energy to my Active, switch to Giratina EX, move the four energy and attack. The same can be done with Sigilyph, Shaymin EX, Darkrai EX, and even Sableye if you think you have that extra turn to use Junk Hunt. This deck also abuses Max Potion and Dark Patch. Since Hydregion's attack is really helpful against most Stage 2's and against Zekrom, Reshiram and Kyurem (unless Eviolited) hes a good attack, and since he discards two Dark Energy, you can use Dark Patch on a bench to get them back and move them back to Hydregion, though I find it better to tank with Darkrai EX and an Eviolite, of if my opponent doesn't have a way to get rid of Special Energy, I tank with Giratina EX with an Eviolite using Dark Trance and Max Potion. It is a Tier 1.5 deck at the least.
4. Give it a rating (out of 10), and explain why you have given it that rating. I give this card a 9/10 for Play ability, the extra point is taken off because of the attack and how you need to be able to get two energy on him every turn, which isn't easy. I love the artwork, it soots this evil Dragon so well, it's almost like I'm playing Hoard, (fun game by the way), so I give it a 10/10.