Mark IV EMP Guardian
They can't immobilize ships on their world. If you don't care about your bordering planets that much, you can ignore it. And they are fairly squishy.
I'll have to take your word on that. Unfortunately, this particular planet borders my designated whipping boy; an EMP Guardian going off there means I'd lose my main military bastion, most likely. And the REF tab seems to be of the impression that my Raid Starships would lose outright to this particular Guardian.
Mark IV Fortress
There are no mkIV fortresses. If there is a normal fort (mkI, II, or III) then you should be able to work around it by not entering its range, then sending multiple bomber waves at it.
Could be either a Mark III and I'm just remembering wrong or a SuperFortress, then. Either way, I distinctly remember that it's very high-level and very powerful.
The issue with not entering its range is that the Ion Cannon Mark IV is under its protection. I'm going to have to enter its area of attack at least once, with my Raid Starships, in order to destroy that before I can send in the Bombers.
If it is a superfort, get ready for pain. Hack until you have bombers I-III, then send wave after wave of them. Expect horrific casualties. (This is assuming that you have neither golems nor spirecraft).
I have Spirecraft enabled, and I have several Mark I and II Implosion Artillery craft. However, I have no Golems as of yet, though they are enabled.
Mark IV Ion Cannon
Orbital Mass Driver
Kill these with raid starships.
My issue with that was that I was under the impression that a Mass Driver is essentially an Ion Cannon for Starships. Now that I know it's just heavily damaging, not instant death, I do plan on doing that, yes.
Core Warhead Interceptor
Not important.
Except in that it cuts off a particular set of options. I agree that it's not particularly threatening, but I felt that it was important to list so that people would know why I wasn't just bombing the place.
Zenith Dyson Sphere
Barracks
The Dyson Sphere is actually your friend in terms of conquering this world. When you destroy the AI command station, the Dyson starts spawning player-friendly gatlings.
The Barrack ships won't do much against your TDLs.
Again, listed more for completeness than anything else. But at least until the planet is under my control, the Dyson Sphere is hostile to me as much as to the AI.
ETA: And thanks to everyone for the advice!