Ok! After reading some thoughts and approaches to the lair mission, I finally started it yesterday.
My around lvl5-6 squad (6 men) consisted of 4 (!) Sniper, a heavy with machine gun and an infiltrator (to find that big alien to kill).
I was lucky only facing one bomb chiron, but i was greeted by 2 sirens directly at the beginning of the level.
My sniper disabled their heads before they could mind control, rendering them useless for the rest of the mission. I placed myself on a plateau from which I was able to focus on the chiron and the screaming thing.
While hammering on them I faced around 8 other aliens (the smaller types) from 3 directions.
I realized that the aliens spawn 2 new one´s nearly every second round.
My tactic was save spamming. After every succesful round, where I did good shots and damage, I saved.
If something went wrong, like being mind controlled, mortar minions destroying a sniper or else, I just loaded again. Needed 3 hours to solve this mission this way and killed approm. 30 aliens! Nearly run out of ammo and they spam endless aliens.
I assume, if you play this on iron man, you lose at least half your squad.
That´s no good game design - sorry!
I agree, you can face two sirens and chirons on round one/two. Ironman would be kind of impossible now 
I tried really to get to that spawning mission goal as fast as possible, but you HAVE to face alot of enemies, else they destroy you from all sides. So you must approach carefully and kill almost all aliens.
Did i mention, that there await you 3 other sirens simultanously when you approach the lair thing ? Man, without 4 snipers I would never have solved this mission.
Agreed, Nests and Lairs are done really bad.
My first Nest simply had no path leading of my Starting Platform… due to this I could not lose (as there was a single tile from which I could see the Spwaner Crabby and kill it while no one could reach me, but it took a massiv amount of turns to find the tile).
Overall I like the base idea of Nests and Lairs, but the way they are done is bullshit. Permanent Respawn is just useless, I had a Lair where I took the “wrong” path, running around the map for 30mins using up all my Ammo and bash the last Sentinel thing to Dust as I had nothing more left… it was not HARD… just time consuming and boring.
My tactic.
5 assaults with “RALLY THE TROOPS” (Each ally recover 1 AP)
1 heavy with Machine gun and jet jump.
Turn 1: locate spawnery. Rectangular hole on other side between fog of war.
Heavy jet jumping to it 3 times. Assaults using RALLY THE TROOPS to recover Heavy AP.
Turn 2: Heavy shooting spawnery with machine gun. Victory.
Yes i know it is not very honorable. But so is permanent respawn.
My main issue with such missions where you are overwhelmed is that once you loose a soldier, everything breaks down. Half of the remaining soldiers panic and therefore do nothing for several turns, and the rest (if any) can’t cope with the numbers. There is no middle ground.
Lol, man, what a system abusing!!! *
Siren is pretty easy to deal with just disable thier head then leave them be, thier will not regenerate after that just keep killing easy target each turn so it will be in constant stat of panic, don’t kill them at that point cuz it won’t respawn. After you disable 3 siren and killed the big guy that shoots cream you Gucci.
No, you´re not Gucci! Why? You forget about the crabman with machine guns AND mortar. 3 from all sides can mortar 2 bombs per turn - have fun! Or they use the machine guns on your weak targets plus the fishmen with their guns crouching in from all sides! They have nice speed…
So, not Gucci ! In addition more Sirens can also spawn. I had 5 Sirens in my mission. When 2 appear simultanously, you need 4 sniper shots to the head to render them useless. Have fun!
And again - pure RNG.
You can have “Gucci” Nests as it seems… where the mission is like stealing candy from a 3year old Child… and some where the 3year old Child was the Child of a damn Neavy Seal who know hunts you down and beats the crap out of you…
Are the Lair’s too hard?
or are they too easy?
Did the first Lair I encounted end in a total squad wipe?
The truth I answer yes to all three questions!
The first lair I encounted had 5 Sirens, 1 Worm launcher, 1 screaming three headed cnt and phck knows how many of those crab and squid men thingys, regardless to say that I originally hated the lair level!
However, there is hope and it doesn’t require resorting to save scumming.
Hint:
-Stealth is your best friend in the lair.
-As much as that 3 headed cnt is just begging you to give it 3 new mouths. DON’T do it.
-Leave the screaming wanker alone, avoid it all together.
-As much as I know you want too just DON’T shoot the screamer, let it sleep.
-Take a different path to the one that the Screamer is on.
-Time is not on your side move quickly and stay in cover.
-Don’t shoot any enimies that have not spotted you or been alerted.
-If you are spotted, shoot to disarm and wound let them bleed.
-Look for a two level deep hole, there will probably be three of them.
–one will have rocks down the bottom; it is NOT that one.
–the other two will not have any rocks, the one that is closest to the screamer is the one.
-By this stage you have probably just gone the long way around the screamer.
-The last trick is to sneak past the worm launcher, don’t worry too much about it as the spaawner is just on the other side of it.
-A couple of grenades or other explosive weapon to crack the shell.
-Then just let loose on the spawner.
If you do this right, you might find that the lairs are too easy 
Good Luck.
With so many enemies spawning attrition warfare is not the way to go.
Having 4 out of 6 units be snipers is definitely counterproductive.
You are fighting on the enemies turf, it makes sense for them to have a huge advantage in a drawn out direct firefight.
Try a mobile strike team with 1 or 2 snipers to take out high value targets.
Also consider if the mobility gained by breaking walls might give you a strategic advantage.
After all it worked well while snipers do the most damage on a long range! That´s what counts after all. Differnet guys, different strats 
here is my tactic:
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1 heavy with HELLCANNON + RAGEBURST + JUMPJET : this one is BOSS KILLER , keep him alive until you have the way to Boss fully cleared. then Jump, and find somewhere stay low, next turn use RAGEBURST with Hellcannon’s full ammo. OTK confirm.
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at least 3 snipers with pierece sniper rifle, they are the task force for the sentiel and Sirens. Mark of Death is good, you should consider one but it need a Lv.7 soldier
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2 tecnicians with Mech Arms, use Field Medic and Tecnician Heal to keep your squads alive. ues Auto Turret as your eyes, rearguards and dummies to attrack enemy’s fire
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1 or 2 squadmate equipped with PDW or handguns(laser is best, it has the best range), they are the guy to kill worms. why handgun? because it can shoot 3 times a turn ( 4 times if you didnt move). Assaults with rifles are OK, but they dont have much margin of errors. one missed and you will have serious trouble.
Bringing a Priest is a good idea (if you have one), the mutant one with three faces can provide 2 will points for everyone at each turn’s beginning.It’s a great buff. Most important is, they can Mindcontrol the worms unlimit times as long as they have enough will points. It useful when you cant kill them all instantly.
anyway, make sure no worms explodes.it really hurts
- 1 squadmate equipped with mounted rocket launter, After scouting, you may find some shortcuts by blast the destructible obstables.
The worst situation is your squad be trapped in the middle by hidden Sirens ( you cant see them, but they keep using AOE willpoint nukes ), so sometimes you need a soldier to lure them out .
my average length to get through in legend difficulty is about 10 ~ 12 turns. if lucky it could be a quick in and out in 10 turns, just keep your eyes on the prize and no need to kill everyone in the map.
You don’t need a Hel cannon to kill Hatchery with Rage Burst. Puny Synedrion laser pistol is enough (50x22=1100 damage). Which means you can jump for 3 AP, walk one 1 AP and use Quick Shot to Rage Burst with 0 AP.
As long as your pistol-armed Heavy/Sniper is within ~25 tiles (20 jump +~5 from 1AP walk) of any tile that has line of sight on target and 10 Will, you’ve won the mission.
For Sirens in Lair missions I found that Warcry is very usefull.
My tactic is to locate the siren and jet pack out of LOS, but in 10 square radius. So i Warcry them behind walls like they do with their psychic scream… Keep all your guys out of LOS in overwatch. Very often the siren will use it’s movement points to go in LOS.
The overwatch will soften her, and she will not have enough action point to use any ability.
Next turn, your heavy will probably be able to point blank shot her or bash her, and any sniper will target head. No problem to repeat this tactic, i won a mission with 3 Sirens without any fancy tech with 2 heavies and only one sniper. The heavies also jumped to the final target and killed it before the rest of the team was overhelmed
Everybody speaks about level 7 ability but in fact i find Warcry at least as powerfull and it’s an early game ability… 3WP 0 Action point cost so usable after a jump… All enemy in 10 square will have -2 AP and will probably be unable to do anything more than going in LOS.
2 shots of shotgun in the face kill the spawnery he is no so tough just bore to find it.
I don’t think anyone is debating how to manage a Siren effectively. It is more so about mission/map design, what spawns, and the degree of difficulty you are dealt with. Sure, over time we all develop tactics, and share our thoughts…
But the game is meant to be fun. After a certain point in the game, difficulty ramps up. I literally sigh and quit depending on what spawns for me on a map. Even Easy mode is punishing. I should not have to build a squad of snipers or heavy hybrids to beat the game.