The most obvious choice is to score goals while the keeper isn't 'coming' yet. The problem is that while going for goal you most likely cannot hit charging defenders, at least using non-metal ball. This requires some shooting skills as there is fairly little space between the goalie's head and woodwork when he stands at the line. From my experience is better to go for a little lob instead of power, something like this:
This way you can get as many goals as you can - the only limit is your ability to kill zombies at the latest possible time (possibly kicking them in the groin or getting head shots when they are just in front of you).
Another way is to score as many goals as you are able when there are no backs left and the keeper is going 1v1. This time you frequently score using metal ball (it will pierce him), or trying to lob the keeper with any other ball (similarly to the method described above). Again, this time the only limit is your ability to obliterate the goalie as late as possible. However, since he is pretty tough and not that slow, you are most likely to get less goals using this method. However, at this point the multiplier should be pretty high so every goal will boost your score significantly.