The erratic camera is not a fail. I have, with much success, learned to use the camera manual rotation during the mercenaries. Headshot/melee kill/manually rotate the camera so you'll be facing the next nearby enemy. The kill sequence changes and, you can't always get it to face where you want it to. That's what the quick shot is for unless there is a safe distance for you to plan your next move.
Just knowing how to handle each wave makes it all easier.
Its a no brainer on urban chaos.
Chain enemies only after collecting all time increases.
Headshot + melee kill for all enemies until the fire fighters appear. Kill then at a distance or shoot their legs or turn em around and shoot the containers in their backs. Armored soldiers...shoot the legs. After a new wave of zombies appears the bloidshots. At that point I just use the strongest weapon I have since the combo is at optimal bonus level. Whoppers are easily killed if a buddy keeps upping the kill count.
Have not yet tried to summon the secret boss though.