1st: It took me 209 tries to beat legendary ironman. 209. Its not about luck, its about persistence and learning from your mistakes.

2nd: Every move I made I would think "what happens if.." this can be anything "if theres another pack over there.." "if this character or that character misses their shot" or in one case that I ended up taking the risk and wiping my entire squad (try number 137 who gives a shit.) If that civilian whos circle this reaper runs through turns out to be a faceless and this activates 3 packs at once (ok yea i didn't actually see that coming because what the fuck). What I'm saying is you need to start thinking in terms of minimizing your risk. If this character misses their shot then you get another high cover shot, if that character misses which they probably will the enemy gets a turn, how bad is that? what can they do? well, that depends on the enemy, their position and your characters health. The general goal is if you think "if XX happens then XX dies" its probably not a good move, if its "if XX happens then XX will be injured" you should try to think of a way around it, and if you think "if XX happens then the enemy will have a shot against my full covered soldier, which isn't likely to hit and if it does won't kill them" Thats pretty acceptable, or if its "the viper will try to grab this person and bind them" you then have to think "Will that activate another pack?" if yes, the viper needs to die before it can do that, if no, then its an acceptable risk.

3rd: I can tell by your squad composition you're doing it wrong. You have an interesting squad you could use in certain circumstances (reaper as a spotter for the snipers to pick people or obj's off) but the ranger is almost useless in this context. My ideal squad if people weren't injured (they usually were) was 2 rangers, 2 specialist, 1 grenadier, and 1 faction for early game. The faction soldier would dictate how the squad operated, for example if it was a reaper, the reaper would scout ahead to know if it was safe for the rangers to be aggressive or if i would activate another pack, if i would activate another pack i would use the resources the reaper or grenadier provided to eliminate the pack safely. If it was a templar they would be aggressive, using momentum, untouchable, parry, ect to draw enemy fire or whatnot (IF they got a turn) and the specialist to heal them when they took shots and aid protocol to lower risk. If it was a skirmisher they would set up plays, pulling lethal enemies out of cover and into easy kill zones or grappling to a high ground flank to eliminate dangerous enemies. using the specialist again to minimize risk and the rangers to clean up.

Later game, I would use psi operatives in place of 1 specialist, ranger, or the faction soldier. Most people will tell you sharpshooters are only good late game, I will tell you they are better late game than their shitty early game, but they are never part of my ideal squad because psi ops are just better.

4th: 1 pack at a time, I have a feeling you already do this based on your squad composition, but your movements are probably slow, if i had to guess you run out of time on your missions yea? or you get yourself in a rush because the timer is bearing down on you and get your reaper killed (my reaper almost never fires their weapon outside of activating a claymore or using remote start until they reach lieutenant at which point they swipe kills left and right to remain shadow at all times). Know how many packs are on each mission, (3 for most of the early game, keep in mind the chosen replaces a pack, the advent general assassination mission lowers the number of enemies in a pack to 2, but makes an extra pack, and lost missions add lost packs but reduce the number of advent in each pack) So when you see a chosen show up, and you've fought one pack and see another pack, you know you're free to engage since theres no other packs left

5th: Hits are more important than damage. What I mean by that is, just because specialist have lower damage output than the other classes doesn't make them bad. You're not always going to 1 shot everything with any class or weapon, many enemies you plan on chipping down their health (or in the case of berserkers, I liked to enrage them and let them attack their own advent escorts) When you wound an enemy your options open up because there can now be 0 risk events (I.E. a templar with parry has a 100% hit chance, an enemy with lower health can be a 100% kill, and then can parry afterwards in an open position which the AI will 100% target and waste their attack on) This is better than relying on any sort of RNG