Eldar Wave Serpant Turret help

Well, the Serpent's turret have a small plastic bit that keeps it from rotating by attaching it in small gaps. Hope you understand, it is kind of hard to explain.
The turret is kind of made not to be constantly rotated, you attach it how you want it and keep it there. I don't glue my turrets so it becomes easier to transport.
About the weapons, well I think you should be able to magnetize them if you use magnetic staves (you attach the weapons on small staves, just like on the Falcon),
Hope it helped

this is really simple to do both for the turret and for the weapons

get some technical lego this almost always contains a little round table (its a square piece with a free moving turn table fixed in the center) and the weapon bases can be shaped to fit inside the peg holes of a upturned piece of lego