There are two tactics to beat Lynch, both require smart players to succeed. Neither player needs to be particularly strong.
First has already been mentioned here. That is to congest the F50 to stop Lynch leading up the guts. Make him go wide.
The extra defensive player needs to be smart because he needs to know when Lynch will be leading, and also watching Lynch to see where he will lead. Ability to read the play is critical. (Jason Cloke is often given this role). If at all possible he needs to get to the marking contest and be third man up.
Secondly, FB cannot play man on man under any circumstances. He needs to play two or three metres off Lynch and time his spoil to perfection. Smart in his heyday was excellent at this. Again the ability to read the play is critical.
Other teams do it and with some success.
Conversely, watch how Saint Kilda play Brisbane this week. They are one of the very few who could afford to go one on one with Lynch by playing Gehrig at FB.