(Sorry about my english)
Okey, the best way to wipe a raid is to kill/CC the main tank (the one tanking the boss) To keep tank alive in a city raid is not a hard thing to do when is't onliny the boss attacking him. But if a dps player starts to nuke tank too it's realy hard and tank should die quit quick.
Other ways to wipe it is to CC main tank (cyclone, sheep, hex, mind control). Then the boss will bounce aront killing healers and dps. That should also wipe the raid.
One thing that i see many ppl do that realy dosen't help a thing is AoE. Like a mage running in and doing arcane explotion, he dosen kill anyone with that and it dosent make any good at all.
Aslo remember this: Many city raids are not orginized at all. The are just some ppl that wannt the bear mount. On my server we are about 10 guys how use to defend the horde citys and we always sucside. We just kill them 1 by 1 coz we are organized and play together.
Hope this tips made some sence and help out some.