Concentrate on the Air threat first and open fire as soon as they first appear as distant specks - they will walk into your bullets. Don't let anyone get on your tail! Drop torpedoes whenver you get chance, don't bother lining up a perfect shot. It can be done!!
Mission 20 is impossible though because there are 3 carriers plus escorts and planes come from 3 different directions almost simultaneously; you will knock down the first 3 or 4 but they overwhelm you in a moment.