There's a couple of tricks: turn the voltage and wirespeed way down and weld in gobular transfer mode. You don't really need penetration of the flange, all you need is enough heat to wet the surface and make the puddle stick. Grind the weld area of the flange first to ensure best adhesion.
Don't weld in the horizontal down position. Weld in vertical down mode, maybe with a little more heat and wirespeed than would be safe in horizontal mode for this thin material. This would be like welding on the side of the pipe, with the joint in a vertical plane.
If you burn through, don't restart in the hot hole you've made. Turn the pipe a bit and continue on. Move quite fast, so that you stay ahead of the puddle. Any droop in the puddle will tend to weld the joint in vertical down mode, instead of just falling away. You can always go around again and patch the holes, grind it a bit, then do a beautification pass.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)