I'm interested in what rating this "beast" in slav and semi-slav is. They are really quite different openings. At sub FIDE 2000 level the semi-slav is very often misunderstood and butchered, i.e the OP is getting spooked by something he doesn't need to worry about.
Ok so it is noted opponent is 1600ish, so would concentrate on knowing what to do on in slav, bishop outside pawn chain type positions.
If a player is a 'beast' in slav and semi-slav, then I think it means that player is good in end games and perhaps prefers closed positions. I would play e4 against such players and even play gambits to make the game sharp and tactical. In short, if his strength is end game, then you should try to finish the game in middle game.