Perhaps try to get a range advantage so you won't have to get near the planet. Broadside Cruisers. Maybe even nothing but Broadside Cruisers. They can sit as far from the planet as possible, and they do decently well against fighters and bombers, as well as big enemy ships. They won't be nearly as robust as your ISDs, but they'll hopefully be able to avoid getting ioned into a coma, then blown up at the enemy's leisure.
If you're going to use ISDs (or Victory frigates), then when one gets hit with an ion shot, abandon it. Move you're other ships away, putting the disabled ship between them and the bulk of the enemy's ships. That way the enemy will hopefully destroy it as they move forward, and right past it, to come after you. This will open up a slot you can use to reinforce with a fresh ship.