I'm assuming you speaking of non-sp decks (though promocroak and ambipom G are great counters to luxray and garchomp respectively.)
But other than that, Donphan, Machamp, heck even Flygon (if Garchomp can't get his energy attachments fast enough) serve as good counters to Luxchomp. Nidoqueen works great too as a soft counter seeing as Luxchomp is like a lot of other SP decks and relies on weakness and cheap KOs to get rid of pokemon (their damage cap hovers around 80-90 due to dragon rush plus crobat drops). So you can heal away all the damage they do with big HP hitters. (lol, you would not believe how hard it is to KO a Gengar lvX when playing luxchomp)