I would use Forretress
Forrestress defense is higher than donphan with slightly less attack than him
Forrestress can learn toxic spikes, spikes, stealth rock and rapid spin , Donphan only knows stelaht and rapid
Forrestress has sturdy incase fire move comes along
Both pokemon though have low Sp def and that becomes a problem
Donphan has a lot more counters than forrstress