Basic Build to maximize crit, sustain and penetration.
This build is good for both solo and dungeon setups, just switch between lover and thief mundus as needed.
Some abilities aren't set in stone.
Reaper's mark reduces bosses resistance but also serves as our burst heal in solo content. If you aren't playing solo you don't need this skill and can use relentless focus instead.
Blood For Blood is a good spammable and isn't as resource intensive as Surprise Attack, our main spammable. Still, if sustain isn't an issue, susbstitute it for a skill of your choice.
Killer's Blade. Important. This is our execute, start spamming this when the enemy is below 25%.
Whirling Blade. Our trash clear. But also useful to deliver burst damage on enemies and bosses below 50%. It gets them to execute range that much quicker.
Rending Slashes. More DoT is always better. If you're struggling with healing sustain, use Blood Craze instead.

Razor Caltrops. Applies Major Breach, slows enemies and does DoT. Really good AoE skill.
Endless Hail. Important. Always keep this up along with caltrops.
Leeching Strikes. Always keep this up. Provides just a little bit of healing while you weave your attacks and helps with sustain.
Power Extraction. Deals damage and boosts your weapon damage. Good.
Reaper's Mark. As mentioned, inflicts enemies with Major Breach and heals you for a good amount when the enemy dies. If you're using caltrops and aren't worried about healing, you can swap out this skill for something like relentless focus or whatever skill of your choice. Simmering Frenzy or Sated Fury are high-risk/high-reward skills you might want to experiment with.

For our ultimate we are using Soul Harvest on our front bar to build ultimate faster, so we can use our main ult Dawnbreaker more often. Use Dawnbreaker whenever it is available.
If you're struggling with sustain you can substitute Soul Harvest with Incapacitating Strike.

Sets you're free to experiment with. You want to come away with a good bit of both penetration and crit by the end of it though.