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.

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