Ronaldo is the runaway top scorer in this season's competition with 15 goals in 10 games and will be hoping to improve on his devastating recent form when his side travel to Munich for the first leg of their semi-final tie.
The Portuguese star helped fire Madrid past Bayern in the quarter-final stage last term, scoring five in a 6-3 aggregate win en route to helping lead Zinedine Zidane's men to another European crown.
The Spaniards are aiming for a third consecutive title this season, but James, who is at Bayern on a two-year loan deal from Madrid, says his side will put up a good fight to stop the current Ballon d'Or holder and his superstar team-mates.
"He's a goal machine and has proven that time and time again," James told TZ of Ronaldo. "He is always there when needed, which is clear in his numbers.
"We hope to have a good day and deliver a top team performance to make sure he does not have his best day.
"You have to be careful with him, but also with the other Madrid players. Real Madrid are not just about Ronaldo, they also have strong players in other positions. It will not be easy, that's for sure."
Madrid came very close to being knocked out in the previous round when they surrendered a first-leg lead to go 3-0 down to Juventus in the return, only for Ronaldo to put his side through with a penalty in the final minute.
The resilience the Blancos showed in the Santiago Bernabeu left James impressed, but the Colombia international feels his current side must not show any fear against them.
"That's Real, they want to win every game and they try that until the very last second," he added. "Nobody expected Juve to go 3-0 up, but in the end Real tried until the end and got the reward for their efforts in the penalty kick. If you try to the end, you will be rewarded for it.
"They always use a different style in the Champions League than they do in La Liga and manage to get the best out of themselves. They know what it takes to fight for this trophy year after year, after all, they have made history in the European Cup and the Champions League. But Bayern don't have to hide. History has shown that Bayern knows how to tackle games against Real Madrid.
"I don't think we should see the two games against Real as revenge for last year, but simply as two more games on the way to the final. The games between these two teams are almost always unique, but we players have to focus on what's really important: to be able to stand up to a team like Real Madrid, we need to be in top form physically and in our game."