Swinub / Piloswine / Mamoswine is likely going to be much more useful than an Onix that cannot become a Steelix. Ground and Ice are two very good types for in-game, and they will help you against the following bosses:
Clair – Just be sure to keep Piloswine away from Gyarados and Kingdra's Water attacks.
Lt. Surge – Spam Earthquake.
Janine – Ice/Ground hits everything but Weezing super effectively.
Blaine - Spam Earthquake.
Will – Ice moves can handle his Xatus and a Rock attack can handle Jynx.
Koga – Spam Earthquake and an Ice attack for Crobat.
Karen – Ice moves for Murkrow, Earthquake for Houndoom.
Lance – Ice all the Dragonites!
Good in-game moves for Mamoswine/Piloswine are Ice Fang, Avalanche, Earthquake, Strength, Rock Smash, Rock Tomb and Rock Slide.