I can only refer to the arcade Twin Cobra, but if the Genesis port is up to scratch on the original then the strategies are this:-
ST5 Boss - Arriving with blue spread, no bombing whatsoever, do an early sweep on the approach then stay on the left side, because the tanks will come from the left first. Stay to the left until the last tank has been killed then pick your moment to tack right and repeat the process on the right side. By the time the right hand side tanks have been destroyed the Boss will be ready to finish off with just a couple more shots.
ST8 Boss - Your method is fine, rinse the first tank out with bombs and concentrated firepower, get rid of it quickly.
ST9 Boss -> The two tanks linked in the middle, keep blue spread and don't go for the kill. Best method is to last them out. Rule#1 - Don't destroy the brace that joins them! If you do they will roam free up n'down and remove any chance of going up and around them. Joined together they are manageable. Don't bother shooting, just dodge. If you find that you are about to be trapped up the top then drop 1 bomb on the side and reverse tack back down the screen. You won't have to last them out for long, they disappear quickly. You will have blue spread for the beginning of ST10.
ST10 Last Boss 'Safe Spot' method - If you can arrive at the Last Boss with at least 4 way blue spread then position yourself dead centre on the approach, dropping 1 or 2 bombs as the scrolling stops to establish the 'safe spot' position, which is your helicopter nose lined up with a grey line just down from the centre of the screen. Once positioned, all bullets from the tanks will miss you (as long as you keep firing your blue spread
)and you can stay in that position until the ST10 Boss dies.