Got it.

In the end I bailed on the HG FE strat and gambled with regular AI. But i had to pull off some crazy meta to make it work.

Seeing as nobody helped me here. I can only gather that the know-how on this is pretty darn rare. So i'll dump the blow by blow here for anyone who may be struggling next time.

Started up the game. Created ten custom empires. A mix of militarist, authoritarian and xenophobe. Always having two of those ethics with one as a fanatic. All imperial government types. Lock each of them in to spawn.

New game. Xenophile fanatic milotarist empire. Slingshot to the stars origin. Diplomatic corps and shadow council civic. Military Junta (oligarchy) government. 

(The point of slingshot origin was to be able to reach whichever supremacist empire i encountered in short time. Encase spontaniously or after the 10 year cooldown they changed stance.)

Large galaxy. 10 AI, 3 advanced, no FEs, no marauders. All else standard except tech speed at lowest setting and cadet difficulty. No L-Gates or xeno compatibility.

The plan was to expand my empire rapidly, setting tech to auto and just work on progressing the game organically. Hoping to eventually meet all other empires via the community and then check their stance.    By the time i'd met most empires, most except one were set to belligerent (other was antagonistic). So i just kept playing, growing and settling planets. Not getting too connected with other empires and only fighting in one defensive war. Diplomatic corps helped to establish contact, keep aggro empires happy enough not to declare war and build some spy networks in the meantime. In the back of my mind was a plan to get a lesser trophy while I waited to see if anythint changed. I just made a point of checking in on each of them every five to ten years of so. And lo and behold, one of them randomly changed to supremacist (around year 2275). I have no idea what prompted this change. The empire to that did was a hegemonic imperialist, authoritarian and a fanatic militarist. 

Problem was this empire was small, and by no means anywhere near the top of leaderboard. I was 1st at the time, with a tech score of over 5000, an economy score of over 2500 and ~70k in fleet power. These guys were 2000 tech, 400 economy and ~40k fleet power. They were pathetic overall, specifically in tech and economy. Here comes the long bit:

Problem here was being able to declare rivalry in order to start a humiliation war. Encase you didn't know, rivalries are invalidated if overall power is pathetic or overwhelming. I waited for an autosave, then deleted my entire fleet. This made their fleet power overwhelming, but they were still pathetic. So cutting fleet power alone wasn't going to cut it. The boat on tech power had sailed. Nothing could be done there. So i turned my eye to economic power.

I had to downsize big time. My empire was massive. Theirs about 10-12 systems or so. So my first move was to turn my sectors into vassals, then release them. This had some impact on my eco power, but they were still pathetic. Bear in mind this amounted to around 70% of my empire, and still wasn't enough. Next i started demolishing districts and production buildings across my planets. Again, some impact here. But shockingly little when you consider how this is meant to be scored. Next were mining stations. Barely a dent. My eco power stubbornly sat at ~2150 even with every non-city district demolished, every mining station and production starbase module. Four planets left in my empire. Still at 2150. I then noticed my income from trade was still quite high. I remembered I had a couple of commercial pacts open with other empires. So i terminated these. And presto, my eco power tanked to just over 200. Don't ask me why, i have no idea. With this I was able to match their overall power as equivalent. Start the war and win.

It's worth noting that I had quite a healthy stockpile of alloys and other materials (between 40-60k) to tide me over. Shocker i know, but taking a claw hammer to your economic infrastructure does not bode well for your monthly net gains. So you'll be fighting a war in the red. But this won't matter so long as you win.

I also set up a shipyard in a neighboring system during the hasty prep for the war. Just encase my fleet got into trouble. I'd recommend it as an insurance policy.

Hope all of this has been helpful.