this looks interesting too: Its slightly different fight in phase 2 between Normal and Heroic, so I'll try to cover both.
First off, try to avoid sending any ranged up into the stands. Some of them may kill the mobs the quickest, but they can still get picked as an Impale target while up there. That puts out unnecessary raid damage and reduces the free damage those Impales put on the Siege. Get the mechanics handled first then worry about killing it as fast as possible
One of the big differences between Normal and Heroic is on Normal all 3 sets of Impales can go out before the Siege poops fire on the floor. On Heroic its energy builds quicker and if its still alive when the 3rd set of marks go out you gonna have a spreading pool of fire to deal with. 1 pool is manageable, 2 is a wipe in most cases.
The timing of the Impale marks and the Siege engines spawning and picking their fixate target are also different from each other. For each Siege that spawns you will have less time to get the mark behind the Siege. I can't give the exact timing on normal as my guild never bothered with him on Normal and I've never had to kite on the normal attempts kills I've done.
But On Heroic the 4th Siege engine to spawn, and it should be your last, will spawn right as a set of Impale marks are going out. But it doesn't pick its Fixate target until after Blackhand actually tosses out the Impales. This is again because the time between marks going out and the Siege spawning gets less and less with each Siege that spawns. So if the two people with marks try to get behind that Siege it will end up Fixating on them and likely killing them.
What you normally do on Heroic is get the first two set of Impales along with ranged DPS on each Siege as it spawns. It needs to die within a couple seconds of the second set of Impales hitting. For the 3rd set of Impales each time you use CDs like BoP or have the tanks soak them with there CD. The damage and DoT can not be immuned, dropped or avoided by anything, but a planned set of Impales is manageable. But with Siege #4 you do the opposite, you soak the first set of Impales than get the next 2 behind the Siege. DPS also ignores the Siege as you burn the boss at this point. The Siege just gets kited and marks 2 and 3 get behind it to prevent the raid damage.
Beyond that its 100% personal responsibility, which is often a nightmare. One person can wreck that entire fight. One other tip for your kiters, the Siege will always spawn on the side of the room furthest away from Blackhand.