Intro:

Build focuses on making use of the force slash proc for rapier ground heavy attacks. At time of writing this build can shred level 155 Bombards with a single proc in seconds.

Essential:

Sacrificial Pressure - not [Primed Pressure Point] as having both Sacrifical mods in place puts heavy attacks into guaranteed orange crit territory for more stable damage output. Damage lost is relatively minute, especially once Killing Blow is taken into account.
Sacrificial Steel - huge increase to crit, which increases slash proc damage
Amalgam Organ Shatter - faster heavy attacks and increased critical damage
Killing Blow - faster heavy attacks and more damage
Corrupt Charge - essentially doubles damage. Since this build is focussed on spaming heavy attacks, the penalty has minimal effect

Basically Essential:

Primed Reach - the longer range makes this much easier to use. Difficult to justify dropping for more damage.
Berseker - Swing speed also matters as well as wind up speed. Can be replaced with Quickening for a less painful first strike

Final Slot:

Life Strike - each hit fills HP back to full, works very nicely against high damage in late game
Condition Overload - 2 extra Forma needed but this is a sizable increase in damage on the second hit
Primed Fury/Quickening - extra Forma needed but will increase attack speed by a large margin