I've done Phinni quite a few times, and I've picked up some tricks along the way.
First, lemme explain the layout of the lair. There are originally 4 swirlspine seahorses around here, one in each of the 4 corners. They are immune to magic. Every time you kill one, there's a chance a guardian swirlspine will pop. Those however are not immune to magic and can be charmed by druids. Then, dead center at the bottom of the lair is Phinni himself.
I've read about fd pulls, but
1) There's not always a monk/sk/necro around when you need one
2) Sometimes it's tricky to pull off
On most of the raids I've attended I was puller. Yep, I'm a 60 shaman, and maybe got unlucky 5% of the time.
Ok, first you start with pulling each swirlspine seahorse. There is a way to single pull them which is almost full proof. Inch up towards the closest one on ledge. Don't swim down, just inch up towards it on towards the tip of that triangle on the first ledge, and eventually he will notice you and proceed to aggro.
Not under any circumstance are you to run back to group until he's actually right in front of you, hitting you. Otherwise, if you run back to group immediatly, he will run around the lair and "warp" onto you along with his friends. If you wait till he comes up to you, he'll pretty much be following a direct path which will NOT aggro the other seahorses.
Then, once he's dead, proceed to do the exact same thing by pulling the seahorse on the left. Again, wait for him to actually be in front of you before pulling him back. Then rinse and repeat by pulling the right swirlspine, and finally the swirlspine all the way in the back.
For these pulls, it is advisable you have a high level puller with relatively high mr. High mr because they cast a stun spell (spinning vortex I think?) which is no good because they do hit relatively hard.
Also, remember how I said that after each swirlspine dies, there's a chance that a guardian will spawn in its place? If you wanna play it real safe, don't pull phinni until he has no more than 2 guardians around him. Repop on swirls and depop on guardians is pretty quick, so the extra waiting time won't be too much - that is, unless you have another guild behind you breathing down your neck.
In any case, once the 4 swirlspines are dead, the only thing left to deal with is Phinni +0-4 guardians. If you have enough druids,chanters, and/or bards, you could handle the 4 adds as they can be charmable, slowable etc. Phinni himself is cake. The only really annoying thing about him is dispell, and if you don't have eb item, you can easily drown.
I have 3 eb items tho, Lodi shield, Amulet of the Grey Wastes, and Ancient Fishing Pole :)