With 20 skill points available, I would go with 6 increase mass (less is useless), 6 gravity well (for maximum pulling), 5 kinetic resovoir (so you don't need pots for leveling) and 1 each in singularity, gravitron slam, and neutron stomp.

This will give you all the skills, just slightly less damage with them. Since damage isn't important for this build, that isn't a big deal. After that, I'd max gravitron slam (since it combos) then singularity.

Let me know what you think when you try it out! Works wonders in numa where mobs can all be pulled!