What level do I give my Onix a metal coat in order for it to evolve in..

1) Any level but I suggest you train it a bit first. I find it's better that way.
2) Well I suggest you get two more pok�mon and level them all up to level 70 at least. You could add Moltres and Arcticuno. If you don't want to then try and get a Fire-type pok�mon at least. A Grass-type pok�mon would also be useful.

1) Any leval then Trade it
2) Yeah but train upto level 80 I would catch aticunno and Moltres and train them to I level 80 and get really good attacks for all of your Pokemon aswell.
Hope this helps keep training and you'll be a Pokemon master

Onix does not Evolve at a certain level it evolves when you give it the metal coat and Trade with some one else, then it should evolve.

I would add both of the other two legendary birds or if you havent got them add pkmn with the same type as them.

Also would you be able to tell us your pkmns attacks.

Any level for Onix to Steelix, but if you want him to get sand-tomb at 52, wait until after to Trade, because he gets crunch if he's a Steelix. That's the only difference in moves, if that was your concern.

1) Your onix can Evolve on any level. You have to Trade him while holding a steel coat.

2)Your team is perfect for fighting at battle tower. Yuor dragonite could use some work though. P.S. All 8 trainers at battle tower have Pokemon on level 72