Unfortunately, Donphan doesn't learn Spikes. Ice Shard is always awesome, just for the countering Garchomp thing. 252Hp/Def, 4Atk is Smogon's recommended spread, though I'm not sure their spread allows you to 2HKO Chomp with Shard, so you may want a bit of attack for that. Impish nature, Leftovers.
BTW, the single rates thread is probable more suitable for this.