And after derby wins over the Dragons and the Scarlets and a narrow home victory over Zebre, Tandy's side are in good form ahead of what should be an enthralling encounter.
"This is as big as it comes. It's a massive game and is one that we can't afford any slip-ups in," he said.
"We've got to be absolutely spot-on in everything we do, our preparation, our attitude and our attention to detail if we are going to get anything from the match.
"We've done well over the last few weeks to put ourselves in contention in the PRO12 and in Europe but we realise we'll need to go up another level this weekend in terms of performance if we want to get the result we need to stay in the competition."
Tandy has named a strong side for Sunday's fixture but has been dealt a blow with the loss of Ashley Beck. The centre suffered ankle ligament damage in the win over the Dragons and will be out for two months.
Beck aside, fellow Welsh internationals Ryan Jones, Richard Hibbard and Dan Biggar all start for what Tandy knows will be anything but an easy game.
"It's a tough group and we are up against teams whose budgets blow us out of the water," he added.
"But we've got a competitive group of players who will give everything for the cause, who never know when they are beaten, and we are looking forward to this contest.
"They'll come down here confident after some good results of their own lately and we know that we are going to have to match them physically.
"They've got the advantage in the pool as it stands but there's everything to play for. What we do know is it's going to be a great occasion."