Did mission 4 on my 2nd attempt (Normal difficulty lvl, I am not good enough to push it up

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Mistake I mind on my 1st attempt: took too much time to build me up, researching unuseful stuff instead of concentrating on logistic and weapons. I spent time growing my colonies and had very weak ships to defend... and also, tried to put some defense on my ships -> this is not useful at all as the DL still blow you up
On the 2nd attempt, I traded technologies with my allies to make them stronger, I focused on weapon and logistic, and finished them with fleets of upgraded Battle Axe ships
Now I am on to the next mission - Apocalypse I think. 1st attempt, same mistake: focused too much on my colonies. Then the DL sent all ships, a swarm overflowing our sectors. Did not have time to give it a 2nd try.
But next time, I will build up the military quickly, colonize 1 or 2 planets less and attack them before they get too strong.
It looks like the DL needs a lot of time to become really strong, you have to rush them more or less.