Shadower Skill: Assassinate

A few years ago I knew how Assassinate worked, though I somehow forgot it.

It's something like

- No charge: 1x mp
- 0.1~2 sec charge: 2x mp
- 2.1~4 sec charge: 3x mp
- 4.1~6 sec charge: 4x mp
- 6.1~8 sec charge: 5x mp
- more then 8 sec charge: 6x mp

But with the coming Union patch next month (Februari) it's all gone, every Shadower skill will be different.

what's the mp thing?
If the shadower patch is coming in June, I don't really want to hold off on leveling things I need to.

So far - I have,
30 Bstep
20 SS (Can't seem to pass a SS30 for some reason)
20 Venom
1 Smokescreen
1 Assassinate
+a bunch of skill points left over
--> I also seem to have a problem with this. In a majority of the guides I've read, they say max bstep > max SS.
As far as I can tell, maxing SS only really saves you some potion usage as well as avoiding knockback some of the time.
Bosses have become easy enough (at least the ones I'm doing right now) that death generally isn't a problem.
Is Ht/CZak so hard that good parties are at risk of death?

How should I max my skills? Suggestions? Reasons?
Also, is Hero's Will worth getting?