[atsync]

I did an Onix submission a few pages back that I think was missed.

Onix (evolve into Steelix): - Mid Tier
Availability: Onix is found in the Relic Path, accessible via Castelia Sewers. It’s somewhat rare (5%), although it is more common in ground shaking spots (20%).
Stats: Onix is well known for its bad stat distribution. 160 Defence seems impressive but 35 HP lets it down somewhat. 70 Speed makes it fast for a Rock type, for whatever that’s worth. Unfortunately Onix has a terrible 45 Attack, and it can’t take unresisted special hits for very long with just 35 HP and 45 Special Defence. Thankfully, Onix improves significantly after evolution. It loses a ton of Speed but becomes more durable and can inflict better damage (although 85 Attack is still a bit low).
Typing: Rock/Ground has its ups and downs, although it isn’t great overall. 4x Water and Grass weaknesses are horrible, and Fighting, Ground, Steel and Ice weaknesses on top of that don’t do it any favours. Still, Onix carries some useful resistances (Fire, Flying, Poison and Rock) along with an Electric immunity, which may be helpful for some teams. Upon evolving to Steelix, it becomes Steel/Ground, which is a much better typing. Steelix gains a ton of resistances and sheds a few weaknesses, all in exchange for its Fire resistance (now a weakness). It’s more than a fair trade!
Movepool: The thing that saves Onix from being truly worthless before evolution is its early access to STAB Dig, which partially makes up for its low Attack. It also gets Strength and STAB Rock attacks, providing it with good coverage. Onix and Steelix actually have wide movepools. Beyond STABs, you get Crunch, Double-Edge and the elemental fangs. Strong coverage really helps make up for the middling Attack stats that Onix and Steelix carry.
Major Battles: Ultimately it depends on when you evolve it. It should be said that Onix may well be the best counter to Elesa in the game because it resists all of her attacks besides Emolga’s weak Pursuit, and it has super-effective STABs for her whole team (and Smack Down can even be used to make Emolga vulnerable to Dig). Onix does less well against Burgh (Steelix is an ok choice for Burgh though) and Clay. Onix should be evolved before Skyla and Steelix can beat Swoobat and Skarmory one-on-one. Aside from Flygon, it does well against Drayden too. It should skip Marlon obviously. Its performance against the elite 4 is good. Aside from Marshall, it can contribute in all battles as long as you keep it away from the wrong opponents. If it has Sturdy, it can also be a decent check to Iris’s Focus Sash + Dragon Dance Haxorus.
Additional Comments: You have the option of evolving Onix right away (backtrack to Virbank Complex and use Magnet Pull to fish out Magnemites until you get one holding Metal Coat) or waiting until Chargestone Cave. Onix/Steelix can be a good choice either way, so don’t worry if you don’t want to wait around to evolve it as soon as possible. It is a good idea to give Onix Eviolite while it's unevolved to buff up its defences. If you are unable to evolve it into Steelix... you probably shouldn't be using it! Sturdy is probably the better of its abilities. Rock Head is only good if you want to run Double-Edge, and Sturdy is more useful overall.

As I said when I posted it, I reckon Onix will need separate entries depending on whether you can evolve or not. Steelix was useful, but using an unevolved Onix for the whole game sounds awful!