How can I improve my Donphan?

Question

Well, I'm currently having a team (ingame) consisting Meganium, Crobat, Herracross, Arcanine, Vaporeon and Donphan. The thing is that I noticed that my Donphan
had lost some of it's glory when I started my Kanto adventure. What I mean by that is that Donphan is starting to suck. Alot. Now, I have an Adamant Rhyhorn on my team which I was originally thought of using, but I was too lazy to train him up to evolve. What should I do? Keep using my Donphan or should I start and train my Rhyhorn?

P.S. I'm playing HG, but unfortunately I can't trade. So no Rhyperior advices.

EDIT:Donphan has Earthquake, Rollout, Thunder Fang an Iron Tail. No Stone Edge TM nor battle frontier move toturs.

Answer 1

I actually think you should keep Donphan because of your teams Rock Weakness(Crobat/Arcanine)and if you switch to Rhyhorn, you would have 2 Pokemon weak towards Ground types. Here is a moveset for Donphan to help you in Kanto.

Fire Fang-For Erika and for Ice types
Seed Bomb-For Misty since you have a Water Weakness
Earthquake-STAB and for Janine, Brock,LT. Surge and Blaine
Rock Slide/Stone Edge-coverage

Answer 2

I see a very easy fix to this problem. The issue with this Donphan is that, indeed, you need a potent Rock-type attack. Since Iron Tail gives mediocre coverage in this moveset, why not give Donphan Defense Curl through a Heart Scale?

Defense Curl doubles the base power of Rollout, going form 30 BP on the first turn, to 60. This makes Rollout stronger than Rock Slide, but a little weaker than Stone Edge.

Without breeding, hours upon hours at the Battle Frontier, or actually getting a Stone Edge TM, I'm afraid Defense Curl is the best way to go. Not that it's bad though, because Rollout is very potent after DC!