Dealing with acid ball usually depends. If the spell has been bound, destroy the object binding it. If you can dodge, good. Jinx and reverse attack work well too. Don't load creatures up with +armor until you can draw/take out acid ball uses. Condition removal is nice, and creatures without armor work nicely too. Depending on the book, fortified position can work wonders. They load up corrode, you/your creature leaves the zone and comes back, back to two armor remaining. This applies to the banner as well.
There isn't a means of corrode proofing creatures, but you can make it fall off by hook or by crook as others have shown.