That said, after Chaos, with Booster maxed and Blood Drain at 3, the duration difference was too great so it was more optimal to cast separately. But after trying a while, I realised that it was too much mental work to keep track of the duration for both. And while I initially thought blood drain was a "nice-to-have", I now realised that it was actually a pretty essential skill as it saved me from death many times and it effectively gives you a 10% hp boost. So my recommendation now is 14 for Booster and 9 for Blood Drain. Cast together as a macro, this gives the best duration of 87 seconds. 9 for Blood Drain also just puts you at 3% which helps for lower damage attacks like Teleport.

It is noticeably faster if you put Blood drain BEFORE Booster as a skill macro. They then cast almost simultaneously. This is one of the main reasons for matching their durations.

You have to decide between Reaper, Adv Dark Chain and Dark Shock. My current recommendation is to keep all 3 at level 13 and save the SP for now until you find a situation where you see a benefit. For example, bump Adv Dark Chain to 20 if you are in a situation where it will help to exactly X hit KO a mob you expect to be training on for a long time. The reason is because all these skills are situational.

I personally prefer to max this as I use it all the time as the opening attack. The additional damage will mean a better chance of 1 or 2 hit KOing a mob. But that's a calculated decision. Sometimes its worth sacrificing a bit of damage just for the fun of spawning more your little reaper buddies :-).

Its theoretically extra free damage. But I find myself not using it much because:

However, reapers are very useful in specific situations:

I currently do not use Dark Shock at all because it cannot consistently 1 hit KO my training mob. I reckon Dark Shock is only worth using if it can 1 hit KO a mob that you normally need 2 blows to kill.

I have found that dark shock can be useful to kill mobs on another platform that is out of reach of Finishing Blow. i.e. if you see a bunch of mobs milling near the edge, Chain+Dark Shock is a good way to clear a few mobs and improve spawn. you can pick up the drops when you cycle back to that platform later on.

Also, after testing Dark Shock at L20 after Chaos, I don't see any improvement in the crit rate. It is supposed to be 45+50=95% but after using it a few times, it feels more like 50%. Either it is bugged or the 50% applies only to damage. Either way, I found that even at L20, it still felt more efficient (and reliable) to kill mobs with Finishing Blow. My suggestion is to test it out when you know free SP resets are being handed out but hold back until you have Dark Genesis maxed. In my opinion, unless there is a situation where a longer range is beneficial, Dark Shock is not worth using unless it can reliably one hit KO a mob. Otherwise, you'd be better off with multiple Finishing blows because they chain together faster.

Advanced Dark Aura is the main skill to max first as it give you the highest damage increase. For the rest of the skills, it would depend on how quickly you get your hands on a skillbook. I would also put one point in Finishing Blow, Twister and Dark Genesis early as they are all useful training skills. One point in Party Shield is a good idea also if you plan on bossing. 

Also, I work on the principle of keeping 5 to 10 SP unused so I can try out skillbooks if I see them cheap. Unfortunately because of hackers, you tend to get cheap skillbooks in batches.

Generally I categorise the skills into 2 groups because the order of adding SP depends on the accessibility of skillbooks.

Advanced Dark Aura

Finishing Blow

Maple Hero

Stance*

Energise*

Dark Genesis

Twister

Party Shield

Advanced Yellow Aura

Hero's Will

* These skills have a higher priority if you plan to be bossing very often.