Thats a very odd question you pose there, and from tactical point of view, not very smart, sorry.
There is no such thing as "a tactic against the strong opponent".
You look at where you excell, and where the opponent excells. Juventus has allways been about tactically neutralising the opposing treath, and trying to play our game in a way that hurts them when we got in possession.
Against barcelona, this would obviously mean we'd go 3-5-2,play it compact and deep, focus primarely on ruling out trough passes, have marchisio breaking up the passing, vidal pressuring, have the ss helping vidal, have matri keeping the defence buzy. generally force them into the physical duel, and when we have possession, make one of the wingbacks race up the field, with 1 cm and the two strikers supporting. the rest should keep tactical position.
Against real we would play more our pressing game, and try to break their defensively week midfield, giving them troubles reaching their forwards. wingbacks pressing to but without risk, cause di maria and ronaldo are lethal.
etc. each opponent has a playstyle we must attempt to counter. If they are better at their game then we are, then we should not "play our game" but focus to disrupt theirs in general.
For this reason its best we try to avoid a bayern munchen. they are very complete, and can handle our physical game. It would require a specialist team like Madrid, or a wildcard like psg or milan (who got a worldclass striker capable of deciding it suddenly, while the team's defence has a chance of holding it)
I hope bayern get dortmund. that will be mad fun