So upon release of EW I decided to go Ironman Impossible no matter what and finish it. I've finished EU on Classic and attempted numerous I/I back then (mostly after watching Beaglerush), but never finished it on that difficulty. It wasn't so much the difficulty itself (although teleporting enemies could really screw you over), but the endgame starts to feel boring when stuff starts to melt so easily. The beginning was horrendous though, where you got oneshotted left and right and had to re-learn everything. Luckily all those hours trying to get through the first months on EU learned me a few tricks that work very well for me:
-High cover = low cover, low cover = death
-Scout cautiously, expect packs to be within 10 tiles of each other
-Be extremely careful with flanking (see above)
-If enemies get a turn, ppl WILL die
-Shoot first, interrogate later (maybe, fu Vahlen!)
-Assaults <3
Put simply: try to murder the aliens before they get a turn, or expect casualties (and often wipe). Assaults are perfect for this playstyle since you basically always get a chance to put a shotgun to their face with Run and Gun. If aliens fall too far back (chances of activating more if you chase), you can still retreat put your guy on Overwatch to make him useful.
Still though, nothing prepared me for EW I/I, as I was getting my ass handed to me again and again within the first 3 months. The increased research times definitely increase the difficulty a lot, since lasers are "gamebreaking" for dealing with those nasty Thin Men (6hp ftl), which you now encounter a lot more and earlier, in part thanks to Portent (which is REALLY nasty with 3 groups close by).
However, I soon discovered that MECs are my new lasers. They're not that awesome as lasers in their default getup, but they can soak a hit or 2 from Thin Men, destroy their cover and oneshot a group with flamethrower. Now don't get me wrong: I love Kinetic Strike as much as the next guy (even more on tall buildings), but at the early stages flamethrower is so much better. I hardly ever have range issues with it and being able to murder 2-3 Thin Men in 1 move carried my ass in the first months.
I still restarted a bunch though, mostly due to panic. Is it just me or are panic lvls super crazy on Impossible? I swear, with every abduction mission you seem guaranteed to lose 2 countries (I've never had those give less than 3 panic). In EU it always seemed to be 2 panic for difficult abductions you "skipped", but not in EW
It's quite sad too when you got a nice game going with some awesome soldiers (Heavy with Bullet Swarm, Low Profile, 80 starting Aim and Mimetic Skin), only to see a game over because every abduction gives you 2 countries in full red
However, I'm now working on my first I/I game where I encountered Sectopods. And boy, do these things want to take revenge for the beatdown they got in EU. It's even worse when they get activated along with a Mechtoid :s Or worse: getting 2 Sectopods and a Mectoid (accompanied by 4 drones and 2 sectoids ofc). My first instinct (ruuuuuunnn!!!!) paid off as I narrowly survived that encounter (in part thanks to a very lucky crit on the Mechtoid and my MEC just having reached proxy mines) with laser tech.
Fast forward a month ingame and I meet 3 Sectopods and a Mechtoid during a terror mission! I would've had to deal with all of them at the same, were it not for my sniper (who just got his plasma rifle) who critted the Mechtoid for 29 damage, insta-gibbing him. Mind you, at this point I still thought I was facing a Sectopod and Mechtoid at the same time, but right after ending my turn, 2 more Sectopods showed up. I was ready to throw my PC out the window, until I noticed my saving grace: Sectopods are afraid of heights (albeit VERY minor height here). I was in the terror map that has a broken highway to the left and many cars laid in front of you. The center of the map in particular had a slightly elevated platform and a big ramp on the right toward some oil containers and a building or something. Anyways, if you know the map I'm talking about: the Sectopods were on that platform near the edge and stopped moving there. The edge is barely enough to grant half cover and there's a small ramp to their left (albeit quite a few steps away), but they seem to have hit a computation error on how to cross this scary (not) height. So ultimately I simply had to keep my Mimetic Heavy perfectly still and snipe away with my SS sniper. It still took ages to kill them and they sometimes decided to use their barrage on my invisible heavy (like wtf?), so it wasn't a perfectly safe situation, but I'm not questioning that lucky break