England star Walmsley was sensational when Saints defeated Leeds 40-6 three weeks ago, racking up an incredible 275m from his 23 carries, most of which invariably left defenders scattered in his wake.
Rhinos are better suited to deal with the giant Yorkshireman in tonight’s semi-final given they will be fielding a stronger pack.
Agar said: “We have to do a job on big Al – he is a world class player.
“But I don’t think it’s a case of cutting the head off the snake and you deal with Al and you stop Saints,” he said.
“I think Tommy Makinson and Mark Percival, at both ends of the field, do a good job for them.
“Jonny Lomax and Lachlan Coote are very experienced pivots and ball players who are exceptionally good at asking questions of your defence and really testing you.
“We know the threats; knowing them and stopping them are two different things.
“We know the levels we are going to have to play at for the opening 25 minutes and the levels we are going to have to get to for 80 minutes for us to be in the contest in the late stages.
“But we are confident we can get to those, too.”
Challenge Cup holders and champions Saints are favourites but their coach Kristian Woolf has been impressed with how the West Yorkshire club defended in the 8-0 play-off win at Wigan.
“They have been doing that for long periods this year,” he said.
“Their performance last week wasn’t a surprise to us. They have a lot of class in the side, they’ve shown their grit all year and been a tough team to play all year.
“They are very deserving to be in the final four and played really well against Wigan.
“We are going to have to be at our best – we are under no illusions there – but we are very confident about what our best looks like.”