I'm just about wrapping up my first victory with my partner in a team game on PBW after a long and challenging 9 months.
I won't go into detail on what specific factors were involved, because really, winning encompasses much more than fleets and battle strategies. See the "When to Attack" thread...
The biggest and baddest player for most of the game was a very good Talisman player. His empire was getting huge. The honest key to victory was planning and pulling off a coordinated surprise attack just before he was in position to do it to me.
I attacked his economic base and raided his planets, bypassing his defensive fleets. He opened warp points on my front lines, which I used against him to launch deep (expendable)strikes behind his lines. While he reacted with damage control, I consolidated my fleets at the front to the best of my ability, even though my ships were far inferior.
I played an Organic race, and pumped out as many Base ships packed with Organic armor (and just a few weapons) as possible. I set their strategies to short/point blank. They acted as fodder. I also configured a few strats to allow some Organic ships to "dance" in and out of combat, taking advantage of the armor buildup each round. But what I lacked was a punch. I got it by building long range strike ships; and my partner helped me out with some of his ships, which were better on the attack.
Best of all, I attacked him with his own ships by insurrecting them slowly and surely... and adding them to my defensive fleets.
Intel.
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Gotta love it. I don't believe anyone mentioned it, but massive Intel superiority is devastating. Truly a key. Broke all his treaties, stole his ships, and robbed resources.
After consistant pressure for many turns, his economy was in shambles and his planets were rioting, halting production. To win, you must be ruthless and relentless, pressing the advantage, and never letting up. It was all down hill from there.