There's a handful of ways that will work, though most are semi-unreliable.
We're looking at T7, W2, with a 3+ save. Wanting a 1st turn kill means you'll need a gun with at least 24" range on it, and more likely 36" or better - the enemy general will rarely obligingly put the thing as far forward as he can.
Mossy Toes' Dark Reaper suggestion is an option, but S5 means you're only wounding on a 6... with BS4, that means you'll on average need to throw out 2 * 6(to wound) * 1.5(to hit) = 18 shots at it. Two shots per Reaper, so a squad of 9 Reapers and you still need to roll at least average. It gets a bit better when you've got an Exarch in there, but that's a lot of points in Dark Reapers shooting at a dinky 50-point gun emplacement. Obviously, we need a bigger gun.
Brightlances can do the trick, wounding on 3+. Which still on average requires (assuming BS4) 4.5 shots, which isn't going to be a cheap proposal either. Sadly, that goes for just about ALL the bigger guns. It takes a good 7 Scatter Lasers (27 shots) on average, which is even worse.
Try and ignore the Toughness then? Rangers might get us somewhere. 2 * 1.5 (BS) * 2 (Sniper) * 3 (Armour Save) = 18 shots. Near two full squads, but that's 'only' a bit over 200 points, which might actually be the cheapest way to kill the emplacement on turn 1... if you manage average rolls.
In short, shooting at the gun? Not the best idea. Focusing down the unit that's holding it is likely easier, but won't actually solve the problem as another unit can take it during his turn and your expensive plane will come in before you can fix THAT unit.
It's almost as if GW MEANT for that Gun emplacement to still be there. And they most likely did. Realistically, that gun is often the only thing the enemy has standing between semi-safety and your Crimson Hunter having free rein to kill things.
However, one thing to take note of: A single quad-gun can be overwhelmed with ease by simply throwing another Flyer into the mix. One of two will still have to jink... but not the other