There are models in Aeldari beyond dark reapers that are good :(
First:
If you're playing with a half Dark Eldar force, that is A LOT of the problem. Index Dark Eldar are terrible. Like "we don't talk about them" level of terrible. If you're playing Dark Eldar out of the Index, I REALLY hope you're not playing Craftworld Eldar out of the same index. They were not in much better shape there. If you are, do yourself a favor and get the codex. The eldar Codex has been one of the biggest triggers for "competitive" players since it was dropped, right there along with Astra Militarum. Dark Eldar gets their Codex today, and it is looking just as good too.
Next you need to understand the core concept of Eldar: they are a specialist army with units who are designed to counter other units and concepts. If you get your silver bullet aimed at the prime targets, you will win. If you cannot get those matchups, though, the game becomes A LOT more difficult.
Fire Dragons shooting at Conscripts won't win you many games. Swooping Hawks trying to hit Primaris isn't going to go far either. I find it generally beneficial to have a splash of a lot of different units just in case I find myself in a position where I need a specialized tool.
The few generalist tools they have are:
Anything Shuriken. Shuriken is so good. Match with Doom for hilarious results. Shuriken can engage just about any target and get some decent results with enough fire.
Psykers. These guys fix just about any game. If you're running Warlocks, maybe try to squeeze in an extra 10 points for the Wound and real Smite for running them as Spiritseers.
Spirit Host. These guys are expensive, but frustrating for an opponent who isn't expecting to run into them. Wraithblades do decent melee, and are tough to remove. Wraithguard with D-Scythes are great for loading up high
AP wounds. Stock Wraithguard are silly good out of Webway Strike against someone who doesn't adequately bubble wrap his army.
Speaking of Webway Strike, 20 Guardians with Shuriken Cannons in Webway Strike can blast a pretty ugly hole in just about any unit. It is fun. Try it if you like Guardians.
Dark Reapers are spectacular against Primaris, too.
Bring a Wave Serpent or two, as well, their Serpent Shield discharges are sneaky useful when cracking tough nuts or hitting an annoying unit with the last couple of wounds to wipe it. I feel that Wave Serpents are a pretty solid counter to Plasma spam, which is what Primaris likes to do... stupid Hellblasters. It takes all the bite of the Overcharge away, as it reduces the 2 damage back down to 1.
Eldar is a sneaky faction that plays better when forcing opponents to play outside their strengths to counter them, or when they are allowed to specialize to counter those opponents.
Dark Eldar? Well, all of that changes today, so I'll reserve my critique on how to do them better and leave that to people who can speed read