Shawne Merriman -- Advice for Raiders ... FIRE EVERYBODY!!

There's only one way to save the Oakland Raiders ... FIRE EVERYONE ... so says ex-NFL star Shawne Merriman, who says the team is "as bad as it gets."

"They need to clean house," Merriman says ... "Bring some people in there who WANT to play football ... and people who want to win."

But the best part about the encounter with the former San Diego Chargers linebacker -- when he took a second to PRAISE the Silver and Black.

"I used to love playing the Raiders ... they sent me to many Pro Bowls. So I can't hate."