Rescue can be unstoppable but there not. In ELO their best cards are banned. They are balanced out. They have high damage cards with low power or high power cards with low damage. Not counting there banned cards marco, alec, and sometimes sledge. They have one on the best bonuses that forces your opponent to pill but they have no card with 8 power becaause that would just be overpowerd. In ELO they wont take you as far as you would like due to most cards having only 6 power and 3-4 damage. Opponents can usually stop your best card an leave you without enough damage to finish them off. SOAs take away their abilities but find it hard to get past the +12 attack. But with marc and alec they are monsters. Those two cards open up kos like crazy. In DTs you lose points for using those cards so most people wont play Rescue in DTs. The staff has done everything possible to weaken the strongest clan.
Nightmare and GHEIST are the hardest to counter when you are playing Rescue, because they usually have pretty good abilities on top of their bonus. Montana and Uppers are stopable since you have two SoBs and, since Cliff was released, two SoAs. Piranas are even matched, since you can stop most of their cards with Elvira (they too lack damage).
All Stars are actually easily beatable, since their cards also have low power (you will have many 4 powered cards against them, but the 12 attack gap is like a 5 pill cushion on pill to pill battles).
Rescue isn't the greatest clan if you are pushing for 1300+ on ELO though, not even close.