This achievement is misleading. It says destroy 250 creatures but what it does NOT say is this can only be done in ranked matches. Only creatures destroyed in online ranked matches will count towards this. The online scene for this game is totally dead right now so you will most likely have to boost this with a friend. The easiest way to do this is have your opponent keep casting creatures as they draw them but not attack with any. Building a deck with creatures that have a 0 in counter-attack rating will speed this up. Just trade wins/kills back and forth. You can easily kill 30-40 or so creatures every match that you win so you both can have this achievement in under 20 matches.