I've yet to clear a T5 quest inside a skyscraper. I haven't been focusing on quests most of my game till about recently now. I'm currently on Tier 2 for my trader with a character level of 133.
But I've had similar experiences with horde armies having legendary enemies + high count inside the wasteland especially at night. It's actually very easy to clean them if you have access to guns without having to retreat. I always have wood frames with me I nerdtower myself up to some random rooftop of an POI, then clean them by clicking on their heads to pick them out one after another with my sniper rifle. They'll be too busy bashing against blocks and running against walls so they are static most of the time.
If I do not have any guns, I simply use my melee weapon and motorcycle. Sometimes I go into a building aswell and stand at a certain spot which will make the AI bash against blocks with high durability. I sledge them through holes and windows. Zombie bears go down quickly with this since they'll be running against the wall and their heads will clip through blocks so you can hit it at that.
If there are no buildings. I drive my motorcycle away from them and divide them. I take on a group consisting of 3-4 zombies with my sledge. If everybody starts to swarm, I drive away a distance then circle them. This is how I pick them apart.
I do not engage zombie bears on foot in the open. I find a ruin in the wasteland to stand on and sledge them from above while they run against the walls and bash it.
If it's in @%$# panic mode, well then just try to cheese him by putting you and himself between trees, maneuver around cars while hitting him and that, somewhere where he can get stuck or take time to move.
I've yet to die in the wasteland with my strategies.