1. Take Multiple detachments / formations from the Coven book for maximum LD shenanigans.
From pg 50: "Freakish spectacle: Enemy units within 12" of one or more units from this detachment suffer a -1 penalty to their LD value"
[Games Workshop Design Studio, Haemonculus Covens, (GW: Nottingham, 2014)]
The keen eyed among you will note that this stipulates the specific detachment. It stands to say then that if you have two Covenite detachments in your force, both will place the LD penalty on an opponent separately. Even better, the formations also have this rule (referring you to pg 50 quoted above). Combine with deepstriking misery armour and tank shocking venoms for maximum troll value.
NB if you want to double down on the fear, throw in some 'terrify' from the Eldar Codex.
2. Adopt a hard 'RAW' approach to Soul Fright.
Main codex pg 105: "At the end of the Shooting phase...unit...must make a Leadership test...Wounds cannot be allocated to models with the Fearless or ATSKNF special rules (any excess Wounds are lost)"
[Games Workshop Design Studio, Codex: Dark Eldar, (GW: Nottingham, 2014)]
The important bit here is that only models with the fearless /ATSKNF special rule are unaffected. There are a few repercussions here.
- Models with the 'Zealot' USR can be affected by the test. The counter argument to this is that their unit automatically passes the 'Leadership' test imposed by Soul Fright. To counter this, point out to your opponent that what it's asking them to do is take a 'Characteristic test' based on their Leadership. Zealot means you automatically pass Morale tests; if GW wanted you to take a Morale test they would have specified. Instead they used the capitalised form of Leadership, which has a specific game meaning.
- If you follow the above, you'll also now see in a big unit with one Fearless model, only the individual model with the Fearless USR is immune. The whole unit may automatically pass Morale, Pinning checks etc., but Fearless does not confer the USR itself to the rest of the unit. Again, this is not a Morale test (which they automatically pass) but a Leadership characteristic test. Hence a big blob of Guardsmen are vulnerable (even if they have a Zealot-priest).
- This also means that Daemons are vulnerable to Soul Fright, as are any units (i.e. Orks over 10 men) that "automatically pass Morale tests". To state it one more time: Soul Fright doesn't ask for a morale test, it asks for a characteristic test based on "Leadership". Say it enough times and you might start believing it yourself.
Here are the key passages from the BRB:
Morale Tests"Morale represents the grit, determination, or (sometimes) plain stupidity of warriors in action. Morale checks are a specific kind of Leadership test."
"Units containing one or more models with the Fearless special rule automatically pass Pinning, Fear, Regroup tests and Morale checks, but cannot Go to Ground and cannot choose to fail a Morale check due to the Our Weapons Are Useless rule. If a unit has Gone to Ground and then gains the Fearless special rule, all the effects of Go to Ground are immediately cancelled."
"A unit containing one or more models with the Zealot special rule automatically passes Pinning, Fear and Regroup tests and Morale checks, but cannot Go to Ground and cannot choose to fail a Morale check due to the Our Weapons Are Useless rule. If a unit gains the Zealot special rule when it has Gone to Ground, all the effects of Go to Ground are immediately cancelled."
[Games Workshop Design Team, Warhammer 40K: The Rules (Digital Edition), (GW: Nottingham, 2014)]
One point it is worth noting here is that the Fearless special rule does talk about the 'unit' gaining fearless in the second highlighted section...
3. Use your Deepstriking Skimmers like Evil Drop Pods without fear!