Grella’s strikes along with goals from Sam Saunders and Shaleum Logan took the Bees a game nearer a return to Wembley, where they lost in last year’s final to Carlisle.
Saunders broke the deadlock after just five minutes of a one-sided affair when he swept home from close range after Clayton Donaldson’s lay-off. Grella then stretched the lead midway through the first half, heading home Saunders’ corner at the near post to open his Brentford account.
The ex-Leeds striker grabbed his second of the game just three minutes after the restart as he finished off a three-man move by slipping the ball beneath onrushing Cherries keeper Shwan Jalal.
Defender Logan made it four-nil after 76 minutes with a deflected drive before Grella headed his hat-trick from Toumani Diagouraga’s cross.
He added his fourth and the Bees’ sixth with seven minutes remaining when he fired home Niall McGinn’s low cross, and may even have made it seven when his injury-time effort cannoned to safety off a post.