Moving Luna around is your best bet to deal with a Dredge. as for directly combatting him, your ult can save you from his ult if you have no way to leave it's AoE

When Io deploys Luna, you can use Broadside to deal big damage or force Io to move her, leaving Luna further from the middle of the field and on a cooldown. With Bulldozer, you can take Luna down with a single rclick, keeping it from the point.