Generally speaking (from my own experience) it got quite hard to qualify on european flights with the now only 500 minimum miles per segment.

So I would guess that it might be cheapest to qualify on a long haul. Here often the USA are cheapest. Two trips should do the trick (if you build in some connections) - and in the low season you can get those for 300-400€ incl. tax each if you also check the offers of CO and NW (e.g. opodo offers NW fares via FRA).

Another alternative could be Australia or New Zealand. Malaysia Airlines flies there and is KLM partner. If you can get early bird fares they are available from 700€ + tax. But beware, AFAIK you don't get miles on the ex ZRH service, you have to fly from FRA or AMS.