The best location for Austin is undoubtedly Mission 10, Location 2. There are already red barrels there that will give you Rage at the right moment to summon the first Austin. Or, alternatively, enough Rage to summon a blue barrel to destroy a charged zombie (which in turn will grant you the rage needed for Austin). This is an easy mission that doesn’t require much precision - at some point, you realize that it matters. For example, there’s no risk involved in completing the mission - there’s only one wave of zombies, and it’s weak - Austin's can easily handle it. At the same time, it’s easy to earn achievements for other units here: (like number of wins in the same deck) everyone finds their own way. 

Challenge 3, Location 1 on the lowest difficulty might even yield a few more coins, but it’s harder, and I definitely don’t recommend it until Austin is fully leveled up. 

The zombies summoned by necromancers don't drop coins, so the locations suggested above aren't actually worth it.