Marshtomp @ Eviolite Lv. 100 -- Torrent
Nature: Bold - EVs: 252 HP / 148 Def / 108 SDef
•Earthquake
•Scald
•Ice Beam
•Stealth Rock
I have found that Tangela is not as good with eviolite as it's evolution is with leftovers. Tangrowth has much more HP, so the difference in damage is negligible and after leftovers Tangrowth will have more HP than Tangela would have had. The only plus to Tangela w/ Eviolite is the added special defense.
Onix has remarkably low Attack, so Double-Edge won't do much. It might still need Sturdy; while its Defense is outright massive, its HP and Sp. Defense are both low (even with Eviolite and Sandstorm on the latter), so a good Surf/Hydro Pump/Grass Knot/Giga Drain will kill it without Sturdy. Finally, try Roar over Dragon Tail. Substitutes block the phazing effect of Dragon Tail, and Onix won't even be breaking those Subs.
Good advice, you can tell I'm a noob. Onix does need an attacking move, otherwise it's just asking to be taunted, so STAB Earthquake would probably be the next choice. Using EQ also allows you to use Sturdy over Rock Head, insuring against OHKOs. Even so, Onix is still a pretty poor choice...
Don't forget the Perish Set (thanks to Bologo):
[SET]
name: Perish Trapper
move1: Perish Song
move2: Mean Look
move3: Roost
move4: FeatherDance / Taunt
item: Evolution Stone
nature: Sassy
ability: Prankster
evs: 252 HP / 4 Def / 252 SpD
ivs: 0 Atk / 1 Spe
I've tried this thing and you can take down even ubers like Mewtwo, Darkrai, etc.