This is the benefit of having a 6'9" 280lb guy who plays like a guard, and can successfully guard SG, SF, and PFs. The C's (and most every team in the league) just don't have any big men quick enough to guard him on the perimeter, which pretty much forces you to match up with them.
Unfortunately, I don't think there is a great answer for it. The way to beat it is to match up on them on defense as best you can (which the C's have done by putting KG/JO at center, and either Green or Pierce at PF), and then just try to execute better than them on a relatively even playing field.
This is the toughest part of the last game. The C's can match up with the Heat, no matter how they play. They can go small if the Heat go small, or go big if the Heat go big. However, if they fail to execute, the Heat just take advantage of mistakes better than any team in the league.