ohhhh just a defensive set?
Honestl donphan can always deal decent damage, it has base 110 attack or something.
for the EV's just use 252HP/252Def/6Atk.
with something like Donphan that has no recovery you really need all the defensive EV's you can get.
Why Seed Bomb? it gets crappy coverage really. Especially since it's only good for hitting Swampy/Quagsire/Gastrodon. Otherwise EQ hits for almost the same damage if seed bomb's only 2x Effective. it's a waste of a moveslot as it doesn't vastly improve donphan's coverage at all.
just use knock off in that slot, as it disrupts ghosts who switch in to block spin, especially rotom. It's also just a generally good move to use on something like donphan, as if you can knock off lefties on the switch it allows you to beat a lot of walls one on one.