Since 2nd Edition, the Eldar have generally held their ground in variety of units, despite sliding up and down the Tiers of capability depending on the meta game - which flip flops from close combat to shooting oriented each edition. The one thing that has stayed the same is their standout technology, speed and fragility.
Through this article I won't be focusing on particular units in a "this one is better than that one" stance, more so how to create a complimentary force at a high level. Within various military groups, doctrine is instilled to keep a sense of "what" is needed at a strategic level, complimented by the "how" in regards to SOPs (Standard Operating Procedures), or tactics.
Once these concepts are fully understood, you will find yourself automatically applying them with little thought. As a result, the decision process in creating a solid strike force will be simplified, meaning you hit the ground with a solid fighting chance (not that the Eldar are in much need of help this edition).
Phase 1 - Play style. So you have already chosen the Eldar as the implement of destruction. Very nice. You are well aware that they are fast, fragile and pack a punch, in street terms they're a "glass cannon". Knowing this, we move into play style. It is important to understand what works for you. Some are a fan of skirmish tactics, others enjoy a hammer & anvil approach, whilst another may enjoy all out frontal assaults.
It is imporant to note that identifying your play style as a general needs to be realised up front. If you don't identify it, then your patterns of play when applied to an army not built to support the commanders intent will crumble as you consistently make bad decisions by default. Basically, this comes down to the "know yourself" principle.
Phase 2 - Army planning. Now we begin building a force around the established play style. What we're aiming for here is to accomplish the following;
a) Prevent unit choice over specialisation
b) Establish appropriate redundancy
c) Establish appropriate unit complimentation (Synergy)
Point "A" is an important one. Have you ever seen an opponent bring a force spamming a particular unit? How often do you see it get face stomped due to encountering an army that see's its one trick pony and denies the army of spam any opportunity to exercise it? Yep, this happens a lot, and all because the army general places all bets on getting that first move off. This is Warhammer 40,000... not chess. A solid balanced strike force with clear intent and synergy will break an opposing force most of the time.
Point "B" is your Devil's Advocate in this sense. While you do not want spam, appropriate redundancy is important. Let me use an example. Say your play style is based on a Saim Hann skirmish/assault strike force, the core composition will be Jetbikes, you WILL have more than one squad, however this does not mean your entire army will comprise of just Jetbikes (hey it can be fun, however don't expect a high win rate). Likewise, you wouldn't just create an army of Wraithknights - you're likely to encounter a sniper heavy army just rubbing its hands together... and "tada!" rock crushes scissors. By all means have more than one squad of your core chosen units, these are going to be objective grabbers, campers etc - the ones that will be your winning MVP's in the game, core to your strategy and tactically versatile.
Point "C" is where the meat resides. Synergy is PARAMOUNT with the Eldar, as effective as overlapping cover fire, their entire stat lines are built around how each unit can support the other. The strategy here employs a high force multiplier. Let me illustrate this, going back to a very simple analogy. You're playing Saim Hann, Jetbikes, Wraithknights, Night Spinners etc. Naturally you don't want your Jetbikes being snotted out of existence, but you need to capture objectives with them, hold up the enemy and prevent them from snagging flanks requiring immediate self lubrication from an imminent railing. Wraithknights are wonderful for getting in the face of threats and can take a beating, your brawlers so to speak. These units will keep nasties at bay while repositioning your Jetbikes. Those that can't be tied up, can be pinned down by using Night Spinners to hinder movement. These units working in concert, supporting each other are enjoying the perks of Synergy. Similarly, a Biel Tan Warhost makes good use of Warp Spiders and Striking Scorpions - Jump, fire, jump back... if things get real, you have close combat melee counter assault units close by to get them out of a pickle. Each work in tandem to minimise loss, while maximising damage output.
So now we have a solid understanding of what contributing factors enable establishment of a solid strike force, at a strategic level.
Phase 3 - Standard Operating Procedures. These are your "if this happens, then do that" field doctrines. Your "go to" response to an "oh crap" moment once deployed. The nature of dice means you will have moments like these and lose units that you DIDN'T want to. The advantage you have is by going through Phase 2, you've mitigated the risk by planning for redundancy, force multipliers and prevented the enemy from exploiting a one trick pony play.
You have units chosen, your synergy has been identified between units. Now the table is set, the terrain is down and you know how you want to use these units. The next step is to Identify objectives, fire lanes, any terrain that hinders movement, open flanks etc - now place your units considering these factors, whilst maintaining pre-determined synergy as part of Phase 2. Typically, a force will break into two parts, with a fallback plan in the instance that something goes catastrophically wrong for one team of units. What ever happens, don't let one units demise ruin your advance - with redundancy and synergy in place, you've mitigated any single weak link in the chain.
Phase 4 - Winning, consistently. Ahhhhh, now we get to dive into Mr.Boyd's OODA loop. The premise of which means you can respond faster to the changing battlefield dynamics than your opponent. If you control the pace of battle, and keep the enemy in a reactive position, you will dominate and subsequently win the game. OODA represents "Observe", "Orient", "Decide" and "Act". If you want to know more and how it has been applied in battle, feel free to Google it, there are many many articles out there. I will however go over it in brief, with the context of Warhammer 40,000. In essence, always keeping in mind the battlefield layout as part of Phase 3, your units should always be shifting for the pre-determined decisive point whilst denying the enemy advantage. The decisive point is that which the final act of battlefield dominance (the turning point, or point of assurance) occurs - The "Checkmate" of 40k.
By already coming prepared mentally with where the synergy lies in your strike force, knowing the force multiplier points on the battlefield, knowing where you need to get to (and where the enemy is trying to access), you can "get in their face", which will in turn prefent any solid tactical response from forming. If the enemy general has to keep rethinking their tactical response every turn, they will never see any long standing strategy come to fruition.
So there you have it, high level strategic and tactical planning. Of course this is a very prescriptive analysis, which can get down to some very granular levels depending on the phase. These granular components are typically around the unit choices, war gear, specific points of synergy, static standard operating procedures etc. There are a myriad of Eldar tactica articles out there, however I've yet to see a good one discussing the high level architecture of a well formed strike force that can be used as a model to fit any army list composition.
This list is by no means definitive, however it should act as a good starting point, given the dependence of Eldar on mitigating losses due to their fragility. Dying race? Not if I can help it.