It's weakness can hold it back, and you'll probably never attack with it, but its ability is really what makes it shine. You can put up to 120 damage on your opponent's board without attacking. Dusknoir can help move the damage around to the most threatening targets. Not being an attack, this lets you make use of Max Potion and maybe Tropical Beach. Unfortunately, you need to be active so catcher can halve your damage output and also you only put damage on the field equal to the number of your opponent's Pokemon... which means yes you can do 120 but if your opponent only has 3 Pokemon out you'll only do 60 (or 30 if they Catcher you). Some variants might find use in Celebi EX for Vibrava's attack or just attacking with Celebi for the 60 + Switch into Flygon's Ability, but Celebi is still a little bit of a liability giving up 2 Prizes with so little HP.
There seems to be many ways to run a Flygon/Dusknoir deck but it's not really getting a whole lot of attention so a standard build really isn't showing up yet.
Like I said there's a bit lacking in this card, even though it can be a solid play. A different weakness, a retreat cost (free would have made such a huge difference) and a usable attack all would have been nice. Its weakness isn't too bad but Shred Rayquaza can be a huge problem in RayEels. Landorus is a good option against Eels that's already pretty good with Dusknoir since for 1 energy you can effectively put 60 anywhere on the board (or 90 if you hit for weakness, just enough to KO an Eelektrik).
Overall, 7/10