I had trouble with a lone sub too - if you lose your anti submarinbe ships there is nothign you can do. I have 4 ships left and the entire airforce and nothign could hurt it. It feels like all naval units should be able to attack the sub but maybe with a weak rating. It's too easy to end up with stalemates where nobody can hurt anybody as the only anti sub units are so fragile they always die first!
Alternatively let aircraft or even subs attack them with a weak chance to kill - just have some way to kill the bloody things!


