Scout the map and nab as many influence outposts as you can. Mandrakes are great for this, they hit hard, move fast, don't get shot on overwatch and can teleport where you need them. Don't waste your resources on Kabalites, start making Wyches, you'll know why later in this guide. You'll want to get as many outposts as you can because your about to hunker down. DE have a painful mechanic where influence effects your loyalty. If you spend your influence unthinkingly, your eco will tank and you will have already lost. Be a aware of this when founding another city. The loyalty hit is painful. I'd say 300 influence is the minimum you should have at any time. Once you start having more than one city, consider double that number. Don't go early on heroes spending up your influence, maybe build the Haemonculus if you have the extra influence for his survivability and infantry heals. Use Archon's inspection sparingly and be careful with your later faction wide ability spendings. Grow your Influence eco while carefully scouting about. Set up ambushes with Mandrakes with Wyches as the killing blows, harass attackers, send them back licking their wounds, put lots of eco into infantry buildings, research, and influence and pray you survive, because once you start reaching mid-end game, your opponents will be the ones praying instead.

At first I greatly struggled with Dark Eldar, keep in mind they are very vulnerable early game and as soon as I founded my 2nd city my loyalty tanked and it took me many turns to get my cities back up to efficient standards. (I also discovered you can toggle off buildings, which I have never had to do for any other faction before.) To say Dark Eldar have loyalty problems is putting it lightly. In Commorragh, the strong rule, and this is no different in Gladius. So how do you avoid the loyalty bomb? Stockpile influence. The more influence the DE have, the more loyalty they have. Always research the early loyalty bonus tech and build a loyalty building if you are feeling secure in your situation but DO NOT make heroes early on. This may sound like a bad idea but the DE have a very powerful, very spammable unit, that you will be using from beginning to end. Though they might not seem like it, DE are 100% a infantry horde army, it's where the meat of their damage is, they are cheap but fragile, and they have an infantry unit to deal with every faction. In short, when building cities, watch your influence, build infantry buildings, and buff your loyalty when you can.

Dark Eldar have a interesting mix of fragile glass cannons. I will list the unit, a little about them and their strategies and how good I think they are with Bad-Mid-Good-Great.

Archon: The Archon is a weird one, he hits decently hard in battle but most of his abilities only take effect if you kill an enemy that turn near him. Meaning if you use your ability, then the enemy manages to survive you wasted your shot and now have a cooldown.
Rating: MID

Succubus: The Succubus cuts like glass and is also made of it. She is so fragile but hits like a truck. Her buffs are pretty good with increasing damage or adjacent units and a attack that hits all enemies around her. and COMBAT DRUGS (more on this in a bit).
Rating: GOOD (But know when to send her in)

Haemonculus: Now were cooking. This guy is great, able to heal units from up to 2 spaces away, and a free no cooldown ability to heal wracks adjacent to him. Makes adjacent units gain Feel No Pain, making fragile units a little bit tougher. Also has a attack power that hits decently hard once leveled enough.
Rating: GREAT

Kabalite Warriors: Garbage, fragile as heck and their damage output is incredibly lackluster compared to what is coming. They fall off hard compared to other factions basic infantry, Termagaunts or guardsmen can wipe them at later stages. They are a total waste compared to the other main battle line.
Rating BAD

Mandrakes: Oh boy these guys are good. A infiltrating, teleporting infantry with a flamer. They have the unique ability to teleport to ANY explore tile for 20 influence. They are decently cheap and are great for setting up ambushes. They can reliable pop light vehicles and kill heroes early game. They hit hard, always have them in groups of 2-3, run them in (since they ignore overwatch) and fry your foes.
Rating: GREAT

Wyches: Alright, here we go, the BEST unit DE have, and possible the BEST unit in the entire game (once buffed). Wyches die to a stiff breeze early game, so I dismissed them. Then I learned about Combat Drugs. Wyches, once they get their party on, hit harder than anything I have seen so far in Gladius. Mandrakes should be your scouters, but Wyches are your battleline. Spam them, pop their Combat Drugs and watch your foes become trophies for your Raiders. The more Combat Drug techs you research, the tougher and harder Wyches hit, they can reliably kill any infantry, monsters, vehicles, buildings, baneblades, knights, whatever, these girls are your melee blenders. Just beware of Flyers.
Rating: THE GOATS

Wracks: Finally some beefy boys. These guys are still fragile like all DE, if a smidge tankier, but put them next to a Haemonculus with his free heal a turn, and these guys turn into absolute tarpits. They mulch light infantry and can gang up on bigger things to wear them down. They shouldnt be your main infantry (as Wyches should be) but they are a great wall to defend cities or as backup if things get dicey.
Rating: GREAT

Hellions: Hmm what to say about Hellions. They are fast moving, skimmers that can jump. They have combat drugs as well to buff them out the wazoo, but even then they seem oddly lackluster compared to Wyches. They are a good flanking or scouting force but I didn't find myself building too many of them.
Rating: GOOD

Kabalite Trueborn: Better than regular Kabalites but still not as good as Wyches. They have a bonus against monstrous creatures, but that is very rare outside of Tyranids. Honestly just go with more Wyches unless you REALLY need the range.
Rating: MID

Scourges: Scourges are a odd spot here, they are fragile jump infantry with EXTREMELY powerful anti-vehicle/building weapons. However they are quite situational to be good only against vehicles/building and are lackluster against anything else. They are a nightmare to Admech but free lunch to Tyranids, build accordingly to your foe.
Rating: GREAT (Depending on who you are fighting)

Incubi: Coolest looking tabletop models, lackluster game performance. Incubi are advertised as the premier melee infantry of the DE, but at the tech you get them, Wyches outperform them with combat drugs by a large margin. They are expensive, don't actually hit that hard, and die quick.
Rating: BAD

Venom: Pretty lackluster, they kill unarmored infantry quick, but thats not exactly hard to do with this faction. Best use I can find for it is a Hero boat, and even then the Raider is far better than it.
Rating: BAD

Reavers: Noice, fast, killy, fragile but cheap, these guys are your melta swarmers. When the battle gets rough, swing these guys in and pop the heavy hitters and rush out. Especially since they get Combat Drugs ;)
Rating: GREAT

Raider: Raiders are awesome, DE get techs where jumping out a Raider gives you bonuses to various things like damage and invulnerable saves, these buffs are so crazy late game you can load them up with Wyches rush them in an unload your drug-crazed cargo to blow up a city in a single turn. Truly crazy late game. They can heal as well (as can all DE vehicles), but for 80 influence you won't be using that early game. In short, they are a force multiplier, you load things into Raiders when you want something really REALLY DEAD.
Rating: GREAT

Ravanger: they are Decent, they kill heavy infantry like nobodies business, can pop vehicles given a turn or two but kind of fall off against heavy vehicles and buildings. Scourges are your answer to the latter. Think of Ravangers as your artillery, they are made to take out the hard targets that your infantry (Barring Wyches) will have a hard time killing.
Rating: GOOD

Cronos: SHLUUURP, PATOOWIE, hear that? That's your tarpit Cronos restoring your Wyches to full health. Cronos are awesome, they are by far one of the Tankiest units you have within this faction. Their noodle arms hit accordingly, (like wet noodles that is) even with Hammer of Wrath they are pretty hilariously weak, but that is not what you use these guys for. Whenever you are about to do a Wych charge, put a Cronos in between them, the Cronos has this amazing ability that after it does damage, it can heal an adjacent units for a large amount of hit points. For maximum beef, put a Cronos and a Haemonculus with Wracks in the middle for nigh-unkillable trollage.
Rating: GREAT (This thing is a vital linchpin, do not sleep on it)

Talos: This thing suuuucks. Not nearly as tanky as the Cronos, it's ranged damage output is lackluster, it's melee damage is terrible compared to literally any Combat Drugs using infantry, and it's expensive. avoid.
Rating: BAD

Tantalus: What to say about the Tantalus, well it's a transport, a big one at that, I think it holds the most units in the game. It's an odd mix or a Raider and Ravanger. It's gun hits a smidge harder than the Ravanger but what you should really be doing is treating it like a linebreaker, load up your best guys, use aethersails to get them where they need to be, unload them and then provide fire support behind the main line. It's expensive and will crumple against any other factions superheavys, so treat it with care, but is a good buffer gunship for your Combat Drug fueled army.
Rating: GOOD

Razorwing Jetfighter: It's neat I suppose, it can heal which makes it pretty tanky by aircraft and DE standards, but it's just a interceptor. Can sometimes kill heavy infantry...sometimes.
Rating: MID

Voidraven Bomber: Like the above can heal, but to be frank, your ground forces do far more damage than it. It can drop a mine every couple turns but it is pitifully weak. Good for softening up an enemy force before they reach your main line, but I didn't find myself counting on it.
Rating: MID

Research the tech that allows ALL infantry to use Combat Drugs, Yup that's right, everything will now hit like 40k ton Warhammer now ;) use that influence you saved up all game to inject the goods and win. DE are insane late game, but the biggest struggle is surviving till then. Pick your battles, lead enemy forces into ambushes, wear them down before picking off the stragglers. To be honest I don't see DE winning early or mid game, they are just too fragile with their slowly building economy. Stockpiling influence is an absolute MUST, you will tank your eco with a bad influence decision be it on abilities or otherwise. Wyches are your game enders, spam them to kingdom come, pump them full of drugs, keep them safe from air units with Scourges, (Trueborn or Razorwings against Tyranids) and they will win you the game. Wyches with all their buffs can 1-shot heroes if you unload them from a Raider and attack. They might not seem like it, but DE are a horde faction, spam your infantry and back them up with vehicles. I wish your luck in your raid Archon, *cheers*