Some cards are very well just have those high attacks and powerful bonuses that you must waste a lot of pillz just to beat them..in most of these cases..one of the best tactics is to let the card through and not waste pillz..an example would be Zatman..he has such a high power and a pretty powerful bonus to boot, and the best way to play against him is to either let the 4damage through of block with a defensive card..that is.."if" you are high on life points and "if" "fury in mind", Zatman can't KO you..otherwise, if you are near a possible KO..you should do something about it, coz if you let it through, it might be the end of you..
IMO what you let through is relative to what the opp has in their hand. if they have four cards that can combine for a 2 hit kO then your in trouble. if they need to hit with at least three cards to get your life lower than theirs then you could afford to let the right one or even two through. Like i said it has to do with what they have in there hand and what you have there is no set rule to let this card or that through .