just play the opening that you play normally. There is no way that your opponent can prepare everything. Playing slav which you dont have experience is more risky.
Slav Defence
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In the variation that pellik gave, I wrote about the positions after 8. Ne5: http://blog.chess.com/mattattack99/sokolov-defense
I'm battling someone tomorrow, and I've decided to try using Queen's Gambit. Unfortunately, he will probably decline it and do the Marshall Defence, which is good for me.
On the off-chance he has done his research and uses the Slav Defence, what are the counters I can use?
[I'm white]