in the old leveling system when you hovered over the skill it would tell you how to level it, but now that skills have changed, there's no hover tooltip explaining how to level them. how do i level solo? no idea. headhunter seems to be kills based, but idk. it's definitely not for headshots as i was gaining head hunter xp for stealthily knocking out a tygerclaw. some make sense like net runner and shinobi. but other aren't as obvious, such as the fore a mentioned solo skill tree.