Frank was at the Emirates Stadium on Tuesday to see England midfielder Rice score two stunning free-kicks in the Gunners' 3-0 Champions League quarter-final first-leg win over Real Madrid.
Rice had never scored a free-kick in his career prior to his first thunderbolt against holders Real.
But Frank was impressed enough to warn his team to avoid conceding cheap fouls within Rice's shooting range.
"Unbelievably impressive performance - I went to the game and it was a great performance from Arsenal, they really deserved to win," he said Friday on the eve of their Premier League London derby game.
"Two fantastic free-kicks from Declan Rice. The first one, clever, bent it around the wall, and the second is an absolute screamer. Top corner. Wow!
"The second one is very difficult to defend - although you can avoid giving a free-kick away. Let's avoid giving soft fouls away, of course.
"The first one, I hope our wall would be a little bit different, so you can't go around that side. But, yeah, let's just avoid fouls."
Brentford have never won at Arsenal in the Premier League, but they could face a weakened Gunners team if boss Mikel Arteta decides to rest players ahead of the second leg in Madrid next week.
Frank insists that second-placed Arsenal will still be formidable opponents regardless of who is in the team.
"I think you can see that Arsenal have got that unbelievable level that they can go to, and they definitely hit that against Real Madrid," he said.
"I think no matter who they put on the pitch they will be strong, and good players."