In this savegame (2.6, very hard), my dropship is holding over one of those missions that appear without an UFO that has been flying there. Let the time run a bit then it appears. When such missions happens, it seems there can be up to 24 aliens (I had 23), as in base attacks or bomber missions. Now, this is just at the start of the campaign. There can also be only 4 aliens, was no problem to win the second time I tried. You need to restart a few times so that there are that many aliens. The first time I did the mission it was (I think) impossible to win, with that kind of team at least. The only tactics should be to have at least a second sniper or more, shoot through the rooftop and slowly wear them down, using bleeding. Constantly stay near or inside the dropship, don't even think of going down or saving any civilians, they will all die. Perhaps block a few of the starcases (fire); sheevar can very well reach the "secure" area inside the dropship when already on the stairs and instakill anything. Don't use the team I have there, there's only one sniper and too much cannonfodder (without armor), since I usually rush down on this map to save as many civilians as possible. Suffocation is a huge problem here; there can only 4 soldiers hide inside of the dropship, aliens can come from all sides every turn, and you need to wait a long time on the rooftop until maybe at least half of the aliens are dead. Everyone who thinks that smoke grenades are overpowered please try this. So, I knew city high rise is very difficult, but that...let it appear with 12 aliens at maximum please, until late game.
EDIT
My second trial with an optimized squad. 13/23 aliens dead, one bleeding. I lost one guy. Looks like I could win, with a lot more patience. 1/10 civilians still alive since he spawned on the top. Certainly a necessary nation happiness decrease. In addition to the things above, IC grenades shot down the staircases with grenade launcher help killing the occasional sheevar, maybe by fire damage even, without reaction fire, as its damage doesn't seem to be affected by the difficulty while they are eagerly trying to get to me.
