A Swing And A Myth: Guide To Liberator

Short answer: AM3 and Insurgency
Long answer: Liberator TPs faster than any other Drk weapon thanks to a combination of Aftermath level 3 and high weapon delay. It comes built-in with Insurgency, which is a surprisingly strong weapon skill which has gotten even more powerful recently. Also it excels when you need to self-skillchain, land spells on resistant targets, spam physical AoE damage, or for whatever reason need to spam magical weapon skills. On top of that its tons of fun to use in general.

Mythic weapons have three levels of Aftermath, but only one of them is truly relevant. Aftermath level 3 (AM3) grants the user 40% chance to attack twice, and 20% chance to attack three times. This does work on weapon skills, but does not stack with double and triple attacks. This makes gearing for Liberator significantly different than gearing for other dark knight weapons, so lets get down to it.

While multiattack builds rule in most other situations, they are somewhat redundant with Liberator's AM3. While it is down you want to build to 3000 TP with a standard issue multiattack set, weaponskill with Insurgency, then swap to a set of gear that prioritizes accuracy and store TP over all other stats. Ideally this will put you into a 3 hit build (Weapon skill plus 3 melee hits).

Its important to note that quadruple/triple/double attack procs from gear take priority over AM3 procs from Liberator. This means that a double attack from gear may override a triple attack from AM3 for instance. QA/TA/DA in gear are still stats that are good to have overall, but their efficacy is diminished. As a rule of thumb, consider gear DA to be worth 20% of it's listed value, gear TA to be worth 60% of its listed value, and QA to be worth about 73% of its listed value. These numbers are not perfectly accurate, but suffice for general use.

To make that easier, I put together a google spreadsheet to calculate just how much Store TP you need in TP and WS sets:
Liberator 3hit build calculator

To use it, make sure you have 528 delay set, then input your WS phase store TP and the number of hits you expect to land with your WS. The blue cell at the bottom will show you how much store TP you need for a 3hit build. Note, this does not assume Samurai sub, so make sure you factor in an extra 15 store TP.

For reference, here is the AM3 set I currently use used when this guide was posted:
ItemSet 360024
The Abyssal Beads +2 are rank25 with store TP +7
The Ankou's mantle is augmented with Store TP +10, dex +20, accuracy +30, and attack +20.
89 total store TP including /Sam sub

This particular gearset is aimed at being able to maintain a 3hit build even while using a single-hit weaponskill, assuming 22 store TP in the WS set, from /Sam and the Abyssal Beads +2.

Now lets look at something more accessible:
ItemSet 360020
This entry level set has 83 store TP with sub and assumes 0 store TP in the WS set, outside of /Sam. It requires all 4 hits of Insurgency to land. For every 3 store TP in gear beyond 83, it becomes tolerant of another hit missing.

Other gear options:
There is a ton of wiggle room when putting together your ideal build. Here are some alternatives:

Grip: bloodrain strap, nepenthe grip, nepenthe grip +1 Utu grip is by far best in slot, but these other options will do in a pinch.

Head: flamma zucchetto +2 So good its worth mentioning twice. Don't bother with anything less.

Neck: combatant's torque or ainia collar

Body: valorous mail Potentially best in slot, depending on augments and your accuracy target.

Hands: odyssean gauntlets or valorous mitts Augmented with at least store TP +6, to a maximum of +8

Rings: chirich ring, chirich ring +1, moonlight ring, petrov ring, Niqmaddu ring Niqmaddu ring is so good its worth building around, despite not having any store TP. Consider something like a dedition earring to offset the store TP you lose by not having store TP in this ring slot.

Earrings: enervating earring, neritic earring, tripudio earring Try to get earrings from one of the sets above, these alternatives mostly suck.

Back: Ankou's mantle Another item worth mentioning twice. Store TP +10, dex +20, accuracy +30, attack +20. Don't bother TPing in anything else.

Waist: ioskeha belt, ioskeha belt +1 Better than sailfi belt +1 if you are a high-buff situation and Last Resort is up. Personally I do enough low-buff content that I prefer to have at least 25% haste in gear at all times, your mileage may vary.

Legs: odyssean cuisses Augmented with at least store TP +3, maximum +8. With the right augments these can be best in slot.

Feet: valorous greaves Augmentable with store TP 1-8. If the extra store TP on these feet make or break your build, these can be a more appropriate than flamma gambieras +2.

Insurgency
Niche WS

Boosted Absorbs!
Solo self skillchains!
Dedicated sleeper for Lowman Dynamis

Did I mention Insurgency is really strong right now?
Insurgency is really strong right now. You should use it, it hits really hard.