Let me first go and say that for all the praise non-DE players give us for our amazing "kill a landraider" darklight weapons... We are shooting an overcosted missle launcher at most tanks. Yes, a darklance has a better chance than a lascannon at popping AV14, but we are shooting a S8 gun at AV 12 and under (which most tanks are) for the same price as Marines Lascannons... and they have a longer range.
That being said how we cope with this problem is we stack up on these guns because... we need to. I swear by the attage "you can never have enough darklight" and by saying that I mean it. My 2k list packs 10 dark lances, 17 blasters, and 4 heat lances on 12 platforms. So I would add as many darklight as you can possibly get without hurting your own playstyle.
That being said we have several options of how to field darklight. IMHO their are only 3 ways trueborn should be fielded, and 2 options are ok for a friendly game.
1st: The Blasterborn- 4 (or 3 if you need to save points) trueborn w/ blasters in a dual venom or raider: Like I said this is the staple of many dark eldar armies out there. Normally fielded in a venom to give the unit duality (trueborn for AT, venom for AI) but a raider can be used for even more AT potenial (and only costs 5 more points w/ flickerfields, or 5 points cheaper without!!!)
2nd: The Lanceborn- 3 trueborn w/ 2 dark lances in dual cannon venom or raider: This is pretty much the same job as the first, but sacrifices amount of shots for range. The problem with this is that while the venom gives some protection, you cant move to use the lances. Most people that run this normally just sit them down in cover somewhere, and have an empty venom or raider to vehicle block and help provide cover, but then they are weak to outflankers. Its not as widely used, but effective non the less.
3rd: Duke's Ride: 4 trueborn w/ 2 cannons, 2 shardcarbines in dual cannon venom: If you decide to use Duke (who is a decent choice alone) This is how you want to run him. Cheap and puts out a ton of shots which use dukes ability. Sure the venom doesnt get the upgraded poison, but your trading the effectiveness for how cheap it is. Also remember duke does have a blast pistol himself
Now for the friendly game options:
Duke's SUV ride: 9 Trueborn w/ 2 cannons, 7 shardcarbines in raider: This gives dukes ability a huge effective boost, the prohibative thing is its price cost, which I dont think will make itself back. It is going to be a big target though, and a raider isnt really that protective...
Assaulty Trueborn: 10 Trueborn w/ CCW Pistol, Dracon w/ Agoniser: Dont underestimate the strength of trueborn on the assault. They rock a ton of attacks, and can whipe most squads they hit easily. The problem being is they cant take a hit worth well... crap... better get them stuck in and get FNP quick.