I am personally a fan of the icharus las cannon on the ADL. I know lots of people hate it but seriously, if you want to punch something out of the sky get the las cannon. It also gives your venomthrope something to do while he floats around giving everyone behind the ADL a 2+ cover save. So would the Quad gun but against a thunderhawk your quad gun will be far less effective, and against just about every other flyer in the game you will get penetrating his unless you roll a 1.

I'm just looking to shoot down a Heldrake, he causes me a lot of damage

well an ADL with a quad gun is around 100 points.