If I may, In some levels the trick is either to find a safe position where you will regenerate shields faster than lose them. In lvl7hard, it is the top left corner for me.
Other levels are generally the top row or bottom corners. Some other levels require you to evade and engage in calculated collisions which again are not enough to kill you. And some bosses (I think lvl8hard?) require you to save up all bombs so you can use those to eliminate bullets and then position yourself in front of the boss, because only bombs won't finish them and if you are too careful, the other spawning enemies will take over once the bombs are used up.
Level3 hard, solo requires you to park in top left but not constantly shoot rockets, only release rocket bursts with arrow-up whenever the UFOs start pushing you out of the corner. Once the boss is there, use bombs to avoid getting shot by the fodder and focus fire on the boss.