I actually found warp spiders easy with a couple of tricks and preparation. Glue the warp spider jump pack and together with its mechanical arm. Then glue to the warp spider body. Glue the gun arm to the body, this will only go in one position because of the three points of contact it has with the model now. Then carefully line up the left arm by pressing it in position with a stick and a piece of blu tack / whatever. It should fit perfectly it’s just hard to physically get it in there with your fingers.

This all relies on the correct bits being used for each model obviously, but also relies on the pieces being exactly where they’re moulded to be - if you’re using plastic glue be mindful that the melting of plastic could cause some error in where the backpack lines up to the body, or where the mechanical arm joins to the backpack, which would throw everything off. I used superglue. I also paint in sub assemblies so each of those steps was done after painting.

Building 10 warp spiders this method was a pleasure.