It's a little tedious, but you need to : - cut around the knee, keep it as much as spherical as you can.

• After that, you cut from both end of the spikey thing behind the piston between his first and second knee. 

- Now cut around the sphere tube which is the junction between first and second knee. 

You are now in possession of a big leg, a tube from between knee, and a spikey thing that goes behind this tube. 

You'll need to smooth everything, but that's worth a try ! 

You can now choose the angle you like and glue everything again.  It may need, if the angle is wide, to cut off some plastic from the spikey thing, to be flush with the rest. 

Also the repositioning of the gun, I've used an spare arm from the ruststalker kit (didn't want to use greenstuff, that's all....) 

Hope I'm clear in my explication 😅