The Ospreys stunned defending champions Munster two weeks ago, running in four tries in a 45-10 thrashing at the Liberty Stadium.
But Tandy believes a step up in quality awaits them when they cross the Irish Sea to Dublin after witnessing Leinster's own demolition job of Ulster in the Heineken Cup final last week.
While they conceded just one try against Munster, Tandy believes Leinster's backline will be far more ruthless in exploiting mistakes.
He said: "They're a team that are in form and have won a lot of games. They've got exciting backs as Munster did, but I didn't feel that we let Munster play.
"We did give the ball up too easily on our exits and we did put ourselves under some unnecessary pressure against them, which I think that we would be punished for against Leinster. We'll definitely be looking to take care of the ball a lot better.
"We've had two weeks to prepare and the boys are in good shape. That is the standard that we set ourselves and we all know that we need to be at that standard or even greater to get the result that we need on Sunday.
"The boys are really focused and they've trained well - we've got two sessions left before the game and this group works so hard and is always trying to better itself.
"There is a realisation that to go and win in Leinster you have to be at the top of your game to be in the contest."
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