I've seen this rhetorical strategy before and I don't know if it has an "official" name, but I like the term "weak man" (in contrast to "straw man"). Straw man means inventing a weak position for your opponent and then attacking that; Weak man means deliberately selecting your opponent's weakest argument and ignoring the rest. It would be kind of like if atheists' entire argument boiled down to "Look how dumb the Westboro Baptist Church people are."