My suggestion would actually be to use the raid starships to target the shuttles under the force field -- they can do so directly, if you didn't know, since they are immune to force fields. So if you use the raid starships to clear out those electric shuttles, then the bombers will have an easy time of it. You can also use something very long-range like sniper turrets, if you give supply to that planet and then build a beachhead. When faced with overwhelming odds under a force field, that's been a tactic of mine in the past in order to take out key targets there.
The turtles are abnormally strongly defended based on the fact that they are, well, turtles and don't attack you. So it would not be that hard in normal circumstances, all those waves that they would have sent against you are instead used on defense. I'd suggest raiding the mass drivers on the adjacent planet and the home planet in order to punch a path for your raid starships, if you can.
Here's a trick you can use to target really large force fields, as well: don't target it directly with your bombers, but target something under it closer to the edge. Your bombers will have the normal hit percent to that unit, but will damage the force field instead since the shots are then blocked by the force field. That's a better way of keeping them far out of range of the stuff under force fields in general, compared to attacking the center of the force field itself, which can be far away and frustratingly hard to hit if there are a bunch of defenders -- I feel ya there.
Hope that helps!
