As far as free-for-all goes, part of the point is that anyone can attack anyone.
One thing my game does, though, is use the Terrain cards as a filter. You target a Terrain, then you target a player. If you can attack the person who owns the Terrain, then you automatically attack that player. Not something that comes up in one-on-one, but in free-for-all, it can come up a lot; what if someone's Terrain has a Combat Reward that you want? Question is, can you beat their defenders?