Dark Colluseast: Null

Also, it does a few different attacks. 2 of them matter: Purge and some dot. Together they are lethal.

The 3-2 and 3-3 versions still purge but dont cast the dot i think. But his normal attack hits harder. I dont know about 3-2, but 3-3 seems significantly harder to me.

To be more detailed, a script for everyone:

All debuffing, HFing and sparking as a team when possible is of course a given, do this at the start and keep doing at when possible. For example when the squad is restabilized after a crisis situation. BM probably should be using most of his genie energie on cloud eruption to allow some flexibility in casting HF when suitable. Barb and sin can cast tangling mire and extreme poison.

Try to spark or AD or use ADlike apoth when boss sais´soon you will be one of us´ to block his purge. Only works if you have low lag and react immediately. If chances for this succeeding prove to be low, dont bother wasting apoth or genie energy and just skip it, espescially the BM is unlikely to have chi in the first place and when he does, he should not waste it if he doesnt think there is a good chance he will infact resist it succesfully. So this is mostly for the sin i suppose as he is tanking mostly anyway.

Barb:
During battle, just focus on filling up chi while casting devour whenever it expires. No need to waste chi on trying to keep agro from sin if you cant.
When sin gets purged: Ream, invoke, (ream again if needed) stand up, rebuf, get down again. When invoke is on cooldown, use dew star of protection or genie skill solid shield. If none available, ask cleric for IHs. When squad is stabilized again, proceed making chi. When others get purged but not sin, that whole procedure may not be needed and just a rebuff will do.
If anyone dies, do the same, take agro and use some kind of protection right away. If for any reason you cannot do so right now, shout wait. Rebuf after rez.

Sin:
Try to use your endless chi to resist the purge.
Recast BP when anyone gets purged or resurrected.
When you do get purged or anyone dies, stop attacking until you are rebuffed.

BM:
Recast bell when anyone gets purged or resurrected.
If sin gets purged or anyone dies, stop attacking so the barb can have aggro.
Focus on managing your chi to cast HF and spark in tandem with sin, even trough crisis.

Cleric:
Keep an eye out for the DOT and purify everyone when needed, starting with yourself, ending with the BM. Keep IH stacked on tanking sin.
When sin get purged, expect barb to take aggro, give him a single IH and recast pdef and mdef buff. Dont waste time on HP MP regen buffs. If barb asks for IHs or his HP seem to go down, give him more IHs as needed. When others but not sin gets purged, you should have a good stack of IHs on sin to allow you to immediately rebuff without issues.
If anyone dies, cast 1 IH on barb and resurrect unless barb shouts wait. After rez, heal as needed and rebuf. If you die, have a resurrect scroll ready. If you die twice: game over.

See how you can fit that into dual clienting. (wrote this after badly reading your post and not noticing that you duo it)

I frequently run this guy between my bm (2.5 aps base) and cleric. I don't have much of a problem with him - BM just runs a simple combo of spark - cyclone - aps attack, and the cleric does what is appropriate for the situation. Pdef debuff, heal, rebuff after purge, purify, or dd. At null, the dog doesn't start the purging until he's past half health, so I usually put the cler in vd until then. If the boss decides to reset aggro and attack the cleric, I have the BM force aggro with river avalanche (which works like flesh ream).

An attentive cleric is most important while fighting this boss because of the purge/ep.