Use your Red Pyro with Shell, Immunity and two purple. So he stays live to cast Defense. Use your ABB with Shell, Immunity and two Health or something. So he can heal. SSD with Immunity and Shell and maybe a bloody Mummy with Immunity and Shell (for Cleanse). In case the other team has a lot of Red in their defense you can try you Blue Soldier with Immunity. If yo going second that is Defense from Red Pyro (Immunity / 5 points) and Shield from Blue Soldier (4 points)
It is still risky against Prisoner teams but the Defense of the Red Pyro certainly helps a lot. Sometimes you will have long fights to grind the others down one by one so don't expect wins in the 2nd round. Quite often I have very close wins and you need a bit of luck that's for sure. Not much else you can do at the moment.