What is the opposite of as-salaam alaikum?

The opposite of "As-salaam alaikum" (peace be upon you) in the sense of a greeting is "Wa alaikum as-salaam" (and peace be upon you too).

It's not really about being an "opposite" but rather a response to the greeting.

Think of it like "hello" and "hello" - they are not opposites, but rather two parts of a conversation.