Ah yes, that would explain it.
The forcefields are stacked, there are 4 of them. Things that normally have bonuses against forcefields don't have such bonuses against them (which allows starship forcefields to be something more than tissue to bombers). They're supposed to have more than 80k health each. The Avenger itself has a base health of 80,000,000, and the shields each have 80,000,000; I think all that gets multiplied by 5 before the game finishes loading, though those might be exceptions.
As for forcefields, it's balanced to be able to cut its way through a full cap of MkI, MkII, and MkIII forcefields with 40 engineers repairing them, in roughly a minute. That is, ah, perhaps a bit excessive. But on the other hand I don't want it to be possible to just indefinitely hold it off with regenerating forcefield bubbles.
Perhaps I could drop it to two shields and give bombers more of a bonus against them, so that they're realistically destroyable, and maybe drop the IV cannon count from 4 to 3... I may nerf it a bit more drastically, actually, so it doesn't take forever to find a level where it's at least possible, and then buff it up a bit to the right level of stark terror.