It took some doing but I finally beat this scenario.
After getting my butt handed to me for the umpteenth time, I realized the problem wasn't *me* not holding onto my planets, it was my teammates not holding on to *their* planets. In past tries, I'd be stingy with my research items, only trading for beneficial techs or denying requests altogether. Eventually the DLs would take over one of my teammates' planets and, to quote Bill Paxton in Aliens, "Game over, man, game over".
So my new approach was to still throw everything I had into Research but this time any new Techs I got, I'd share with all three of my teammates. I'd try to get something from them in return but throughout most of the game I was way ahead of them technologically so I'd try to get as many BCs or Influence points as I could, even if it was only 20 BCs or a couple hundred IPs (I wasn't going to give the stuff away). By the end of the game I was hemorrhaging BCs so badly that getting even a couple hundred BCs for my techs kept me out of the red.
After a little while the DLs were trying to take my planet that was closest to them but they kept sending a troop carrier all by its lonesome. All I did was create the largest ship I could with as many drives as I could fit on it leaving room for one small/wimpy laser. That way I could outrun the wave of fighters, frigates, etc trying to kill my one defense ship but be close enough to destroy the unarmed troop ship when it parked itself one or two spaces away from my planet. I did this for quite a while.
Toward the end of the game the AI started to get smarter and more prolific ship-wise and I had to shift over to military spending. I built one of those planetary defense buildings that double the hps of your ships but nothing less than a battleship sized ship was able to stand up to what the DLs had. That was where I built my fleet.
But while I was building a fleet my teammates were busy researching...and being stingy! All that time I carried them technologically to keep them alive - ingrates! On top of that, the AI finally figured out that sending an unarmed troopship wasn't the brightest of moves and started to group it with an escort or three. My retreat & strike tactic had run its course. But by that time their ships had progressed to the point where they were able to move 14 spaces per turn anyway, which I couldn't outrun. Didn't really matter though, by that time I'd built 6 battleships loaded with phasors, shields, & armor. Slowly, the DLs went from only losing one or two hps while blowing the crap out of my fleet, to losing all of their ships while my orbiting ships were left largely intact. The worm had turned. Couldn't get more than 2 into a fleet but it ended up being enough to take out their orbiting defense ships (just barely though).
I was surprised to find their planet population to be 10 (!!) and they went down surprisingly easy, even though I did a traditional combat (i.e. no tidal, no mass drivers, etc). When they'd land 10 on my planets they'd take out anywhere from 3000 to 6000 of my population.
Anyway, I'm glad that's over.
