The quickest way I've found to do this is go into Giddy Park on PRIVATE WITH AI settings.
In my opinion the best thing to do is do this with a friend, and tie it with the following achievement:
You both want to go on top of the ride, that goes up and down with a circular moving platform. It is near the Zombie-side spawn.
When both of you are on top of this ride, take in turns to trigger your All-Star Dummy Shield or Citron Peel Shield, and then shoot it with the opposing character's weapon.
Not only will you get these achievements super-quickly, you get MASSIVE amounts of XP for blocking with these abilities. At the time of writing, you get multiple awards of 50XP with each clip of fire from each character. Make sure you have the 'Leveling Up' upgrade selected on each character. This is available from Level 1 with each character. If you do not have the upgrade, you'll only get 40XP with each XP award.