by losing every war... badly.
Suicidal difficulty with all the AI's possible on a medium map.
The Yor declare war early but I'm able to hold them off for about 40 turns with massed crappy ships. They have to run through the Krynn too thankfully, so I never see transports. Eventually they brake through, but I am able to sell most of my starbases to the Iconians for sweet techs (they're friendly and strong), before they're destroyed. With no shipyards either, its only a matter of time until before the Yor start taking planets.
Suddenly though, the Altarians come to my rescue by destroying the crap out of the Yor. They were probably at war for a bit, but the massive power difference made the Yor surrender pretty quickly. I hadn't lost anything valuable, so I start rebuilding like a madman; shipyards, starbases, constructors, etc. At this point, I only have about 100 power, to the Altarians 4000, and the next up are the Iconians with 400.
The benevolent Altarians declare war on pretty much everybody simultaneously, about 15 turns into my golden age of rebuilding my shit. So I'm back to having absolutely nothing except my planets, but the Altariens suddenly go over 5000 power. This (apparently) makes your power rating drop to 0. They promptly ask for peace, which I sure as hell accept. Everybody does, except the idiot Iconians, who think they can win now.
I didn't ask any questions at this point, and go right back to rebuilding, with plans on taking over the very weak Iridium next door, who are somehow weaker than me. I am able to build a respectable fleet rather quickly, as I had researched high tier weapon, hull, and defense techs, but the fleet is slow like molasses since I didn't get engines. I'm about to start sending my ships towards the Iridium when the Altarians, still with 0 power, surrender to the Iconians, who only have 2 planets left called Kona and Drengia.
I don't rethink my war with the Iridium to much, as the Iconians have gone to 0 power, and will likely not initiate or interfere with any wars. I press turn and the Altarians are gone for good. A couple of turns later, my ships are finally in position, when the Iconians surrender... Because of their war with the Drengin, who are even weaker than the Iridium!
Thankfully, the brilliant Iconians surrendered to me since we were buddies. I inherited the entire map except a few Drengin, Iridium and Krynn planets...
That's how you do it.
(I'm pretty sleepy, so sorry for the mistakes or unclear bits)