The Bills and Pats have a HUGE game this Sunday in Buffalo. Both teams are undefeated at 3-0 ... and Bills mafia is STARVING for a win against Belichick and Brady.

FYI ... Patriots are 32-4 against Bills since 2001 -- the year Tom Brady took over starting duties.

We reached out to Bruce to see if the all-time sack leader had any advice for his old team in beating the Pats, and he says the key is pain.

"If you're able to every other series put him on his back, you hit him. I'm talking about hard, legal hits. That sticks in a player's mind."

Smith says the ultimate goal is to rattle Brady, making him so upset that he TURNS ON HIS OWN TEAM.

"They get flustered, they start venting to their offensive line. That causes dissension in the ranks."

BTW ... the Bills are a 7-point underdog AT HOME, so Vegas obviously thinks this is easier said than done.