The only super-effective type against Garchomp's typing is Ice. Focus Sash is nice, but that will leave your Lucario frail. You could, however, give him a Focus Band. It's like Focus Sash, but with a twist. Your Pokemon may survive a KO (whether it's at full HP or not) but with a toss. It largely depends on luck, but since Garchomp is almost always 2HKO'd by Ice attacks, this item will make it 3 hits sometimes.
Though, not sure but, I think an Ice-attack from an Ice Poke will OHKO due to its being STAB. Not sure though.