here is how i've beat the subs
the main thing was to move my ptero to the island beach
this greatly reduced possible attack points - the subs can't attack from the ground
(the position was not the best, but it worked)
other thing is to move the large group of killerwhales (those ones which are not attacked at first round) to the fortified fleet
i've started to move the carrier only when i've devastated all nearby subs
many of my shielding killerwhales vanished too and only option was to run away
at the end i had nearly dead ptero and two killerwhales