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!