I will tell you how I did it. I found one (actually two) in Xenon Core. I spammed hammers at all the other capitals (killed 2 K's, 2 J's and countless Q's) enough to clear the gate. Then I jumped in my Guppy and Hyperion with an additional 8 five-star marines. I burned away from the center of the sector, while shooting at the I to get its attention. Once I was out 30km form him, I matched speed, and killed off any fighters, P's, or Q's that spawned to help him. Once he was sufficiently far from the center of the sector (away from where any spawning Xenon could kill him once I took control), I reloaded with the missiles from the Guppy (It carries an additional 450 hammers and 600 flails) and worked down the shields of the I. I was around 25km from it the first time and 20km from it the second time.
Oh, I forgot to mention, the I can and will barrage Firestorm torpedoes at you. Make sure you shoot them down! They will kill you. He usually fires 3 at a time. Once he runs out, he will fire hornets at an ungodly rate. These aren't such a big deal, just point you rear end at them and let your flak take them out (assuming you are using a Cobra)
On to the boarding. It took me a little while to figure out the timing of it. The shields recharge very fast, so 1 flail a second or so is needed to keep them down. While the marines are in route, transfer the extra 8 marines from the Hyperion, and keep running 1 flail a second, and shooting a flail at each firestorm he fires. Once the marines land, launch the second set of marines; they should meet up with the first around deck 3-4. Save before the first set make it in. This is the save you will need to come back to if you don't have 16 or more marines once you make it through all 5 decks.
If you don't have at least 16 marines still alive when the 5th deck is clear, they will all die. I had 14 alive when the 5th deck was clear, and they all died instantly. You will need at least 16 alive after all 5 decks. Once they start hacking you can save, and come back if you want to get different drops, but at that point the ship is yours.
I killed the second I in the core, because it had hull damage. It respawned in Xenon sector 695. This is where I took the second I. Same method, burning away until it was sufficiently far and all the supporting fleet was dead.
pic of the two I's.
Pic of the I in action.
- martimus