I tried to do that actually, but they have a fighter group that disable your ship when they got the hull. So when I rush with my onslaught to the enemy Astrals, my ship just gets disabled then I get absolutely demolished by a rain of missiles. 
Automated Repair Unit, Resistant Flux Conduits, Armored Weapon Mounts may help with that. If you're concerned about weapon turn speed due to Armored Weapon mounts, also throw on Advanced turret gyros, but probably won't be necessary.
Overall, those 3 hull mods will get you half damage from Ion weapons, half time repairs, and doubled effective hit points for weapons. If you're going heavy on flak on an Onslaught, that shouldn't be too bad of a hull mod investment. Certainly Integrated Point Defense AI and Integrated Targeting Unit (or Dedicated targeting Core) are must haves. If you don't have the Integrated Point Defense AI, then flares from broadswords and the like are just going to shut your defenses down.
The Ion Beam on Xyphos should only have a range of 1000. You could put 2-3 Hypervelocity drivers on the 3 front most mounts which combined with ITU or DTC, which will out range them. You should be able to target them and take them out, reducing the amount of ion damage coming in. They're respawn, but hopefully not too fast. Given the Onslaught already comes with Thermal Pulse cannons, which are enough to deal with an Astral's armor, you probably just need a Devastator in Front. Probably just put single Flak in the side large slots. Vulcans in all the smalls.
You can either skip missiles, or maybe throw Proximity Charge Launchers on. Those... Are surprisingly effective against massed fighters.
What skills and ships do you have available? Do you have all the hull mod and weapon access you might need?
