Honestly I think his best build is mortal strike build.
Shade at 1, heal from skeletons at 7 and then the Q upgrades at 4,13, 16, and 20. Typically I go for frost mages if they've only got 1 front liner, but if they have multiple melee and you're getting dived then I guess poison nova... But that doesn't happen to me much because people are counter picking with range not melee.
I think you can tech in backlash at 1, harvest vitality at 7, vulnerability at 16, and bone spear at 20. Depending on comps. If they've got a chogall go for all the % based damage and fuck him up.
I really don't think the skeleton build is that great unless your dedicated to split push on a big map... The idea being that someone comes to stop you and you just man fight them doing a lot of damage in lane with the skeleton build. But typically if I'm split pushing they come with 2 or 3 and then it's just better to run anyway..... Also, it's generally better to team fight then split push so I tend to like scythe build better for that reason.
Edit: I think li Ming is pretty bad in lane and gets crushed by xul. So far the best lane counters I've seen have been Johanna, Jaina, and KT. Johanna can just knights vs pawn forever and soak, Jaina can kill you if your not playing safe but will run out of mana, KT can also kill you and has less mana issues. Still it's easy to not lose these lanes as Xul, they just been the most successful at stopping me from pushing the towers in.