Not sure where to progress with 8 unused skill points

My current character is level 40, and starting in act 4, and have unused skill points of 8 and can refund of up to 6 skill points.

Currently im using the caustic arrow and sharpnell ballista, while the build is fun, it is now getting harder in act 4 since im not sure where should i progress my build and which gem should i use.

Please advise where should i progress my passive and gem.