If you're getting hit 10-20 times in a row then it means that the enemy is getting the time to take 10-20 shots at you... which seems far too much; the battle should be over much sooner almost always - you can't afford attrition battles since any damage costs you funds that are required for upgrades.

But starting with hard difficulty probably was the mistake - unlike other games, it's not something you do before trying other things, it's something to do after you've tried other things and found out a strong strategy that works for you.