Editing Warhammer 40,000/9th Edition Tactics/Thousand Sons (section)

====Eldar====
*'''Craftworld Eldar/Aeldari''': A fast, psyker-heavy army that can be deceptively versatile - ranged and melee are both valid options for them and they tend to rely on units that have a designated purpose that they excel in, though they can also be built in other ways. Jetbikes that will cross the board in a turn with little effort, or the very durable Wraith units that are just as slow-moving as you are, to (smallish) hordes of guardians wielding shuriken catapults supported by a compliment of artillery and tanks. However, one universal thing about them is that they will be quite reliant on psykers to make their units shine. You've still got a decent chance to out-cast them, given all their psykers are either classified as HQs or their very contested Elites section. However, unlike your own sorcerers, their big boy psykers get two denies a piece and are immune to perils by default. 
Add on top of that the fact that they will easily outrun you, aspect warriors that can carve through your marines with little difficulty, and a myriad of other shenanigans and you might be in for a rough one. 

However, their infantry are universally a squishy T3 with an average save of 4+ that need to get relatively close to do their damage. Their aspect warriors also tend to perform poorly outside of their designated roles. If you can force them into unfavorable positions, massed boltguns and flamers will make short work of their forces. Their psyker support is also very much not built to take fights, catch one in melee with a squad of basic rubrics and it's very much probable they'll die. They very much aren't hurting for AP which is what typically lets them carve their way through MEQs, your army-wide invuln and the prevalence of D1 weapons on their infantry units means that you'll actually be able to still stand up to 'em when they do close the gap much better than your vanilla CSM counterparts.

*'''Dark Eldar/Drukhari''': Edgy, spiky elves that are three factions in one. No, really. You've got shooty kabals, fast and stabby cults and the tough (by elf standards) covens. Much the same applies to these guys as it does craftworlds - they are faster than you and have little trouble killing more elite units. Compared to their cousins, they are somewhat quicker, a bit more frail and also a bit cheaper and also gain power the longer the battle goes on. 

Generally speaking, these guys tend to ride into combat amassed on relatively frail skimmer transports that go pretty damn fast, which they can also shoot out of. Popping these first will be the big step to defeating them, as once you get past that hull the squishy bodies inside will have to book it to cover to not get liquefied by your shots - the same statline of T3 bodies and 4+ saves still applies here. Counterplay does depend on what they play, though typically they'll wanna get into melee with you.
*Kabals are gonna typically hang back and pepper you with poisoned shots - meaning they wound on 4+ against anything that isn't a vehicle regardless of toughness. Not that this is scary to your marines at all, as they have no AP and do a single point of damage and cultists aren't worth crying over. No, these guys will be the biggest threat to your vehicles and daemon engines, firing a myriad of blasters, dark lances and other anti-tank weaponry that'll make short work of them. Sure, they can pack disintegrator cannons along which are very much an anti-elite choice but more often than not, they'll rely on the other subfactions or their incubi mercenaries to do that for them in melee. Watch out for those ones, they're scary good at their job and also get power armour for a 3+ save
*Cults will ruin your day, with their basic Wyches moving a staggering 8" and their other units going much quicker than that, all while crapping out a bunch of melee attacks. They also shoot up drugs before combat to make things... interesting. These can do a myriad of things; make them tougher, hit harder, go faster, or shoot a little better. They have absolutely crap armour, charging into battle almost naked and relying on their superior reflexes to dodge instead. As you might imagine, it's easier to dodge a blade than a bullet, meaning that these mostly-ladies are even more fucked than any of their other kin if they're caught out in the open to be shot at. Pop their transports with your big guns first and foremost, put your other shots into their jetbikes and hoverboard riders as much as you can. That will buy you some time to then shoot their rank and file (hopefully) before they can close in on you.
*Covens are also melee focused, though they'll have a much better time shrugging your attacks off with their basic infantry being T4 and a faction-wide feel no pain that will give them a shot versus your psychic damage. The scary things are their pain engines though, relatively quick flying scorpions that are mostly gonna wanna get close so they can butcher their way through your stuff, though they can also be kitted out for shooting things too. They are only T6 however, meaning that heavy weapons can still cut through them efficiently. Hell, a buffed squad of terminators can probably punch them back just fine too.

*'''Harlequins''': Are you scared of clowns? Odds are, you should be. An uncommon faction to come across (though competitively they've actually been doing well so maybe not as much as it was in years past) that is not forgiving for new players to pick up. If your opponent is playing them, they'll almost certainly have a good idea of what they're doing. Much like the other eldar, they are blisteringly fast and rely on an army-wide 4++ to protect their squishy T3 bodies (noticing a pattern yet?) to survive. They rely on the sheer amount of trickery they can pull off to win. Did you think your character was safe because there was a 10-man squad of terminators in front of him? Tough shit, they can jump over models with ease in their move phase and land on the other side for an easy charge where they'll proceed to blend him into paste with their melee damage. They've got numerous stratagems that'll let them get anywhere on the board, shut off rerolls against them and more.

Your best bet is to keep your distance as best you can. Unlike dark eldar, popping their transports will be much harder and you're better served on waiting until they get out to shoot the elves inside. They've limited defence against mortal wounds though, take out their Shadowseers and they'll lose any sort of denies or auras that'll save them from smites. Once they start taking losses, they lose steam big time and their turns become quite anemic. If you're alive at this point, you can easily snag victory.