I have been hesitant to bring this up until I saw for sure that it still worked so well on painful difficulty as it did on the lesser modes.
I haven't put any sort of defence on any of my ships for 8 or 9 games now.
Not one deflector, not one armor, not one point defence, nothing, zilch, nada.
I take the time I would have spent researching those techs, and instead get ahead in the warp drive department.
I build small ships early, then when I have good engines and beam weapons, I switch to medium. I haven't tried this with larger hulls and suspect it wont work as well with them.
I end up with fast heavily armed ships.... they have much more offence because they have no defence taking up space. They get to shoot first because they have great engines. (At least that's how I understand it)
If you get to shoot first, take advantage of it and kill them with the first shot.
The AI has of course researched and used shields to deflect my beam weapons, but the beam weapon seems to overpower the defences every time.... and on the first shot I might add.
I am sure this will work with mass drivers and missiles as well, just using beams as an example.
In the entire course of a game, a fleet with 10 of those ships will lose MAYBE 1 of its units, reducing it to 9 ships.
During this time, this fleet will destroy hundreds of enemy fleets without the enemy fleet ever firing a shot. Sometimes, every now and then, they might hit one of my ships for a few points... meanwhile the AI fleet is wiped out on my first salvo.
I know people will come and argue how that wont work in every scenario, but it has worked in every scenario I have had since I started using the tactic 8 or 9 games back.
Not just "worked"... it has been stunningly effective. Amazingly effective.
All I can say is, try it once, see if it works for you.