I killed Vanstrom Klause, arranger of the blood tithes across Morytania and arch-enemy of the Myreque, in a well planned assassination mission that took place over the course of several days, infiltrating darkmeyer, gaining a costume that made me look like a vampyre (it didn't, but whatever) and getting to know the vampyres until they trusted me as one of their own before nicking some sacred wood and making weapons with it to kill him.
In the interests of recording information for posterity and helping anyone else that passes along the way, here is what I learnt about how to defeat him along with a few bits of info I wasn't told, but that was particularly helpful.
Here was my method:
Gear: Karil's top, coif, and skirt. Dragon boots. No ring (ring of life does more harm than good, I would have been teleported the time I won and it was very easy to be teleported while still having a chance to win. Not hard to get to your grave). Amulet of glory. Ardougne cloak. Blessed spirit shield. Bloodwood stakes.
Attack style: rapid. You get damage in much faster, and it also lines you up nicely so that you always have room to turn for his darkness and blood attacks. I stood by the desk and got into a good routine there.
Inventory: holy water, medallion, prayer renewal potion, ranging potion, 2x prayer potion, and the rest rocktails. 12 Rocktails in terrorbird as well. I did try brews, but they proved an expensive waste of time: although they had more health overall, my chief problem was suriviving rapid damage, which rocktails are better for. You also need to drink restores with brews, and it's just a pain generally. If I can get them to work, I'll give them a go, but for now they aren't worth the time to me. Sold the ones I had left. I had some rocktails left over.
Boosts etc: I used the vampyrism aura (ha), eagle eye, and a ranging potion. I might have used the firemaking hp boost if I'd remembered, Oo'glog spa is unnecessary (my agility was 66, and you don't have to run too far in the flying attack: you must stay reasonably near to deal damage). Protect from range is also unnecessary, with Karil's blocking every magical hit that wouldn't have gone through prayer anyway, like blood.
Notes on Vanstrom's attacks:
- During the flying attack, you can get great damage done. He fires two bombs, one under your feet, another in a seemingly random location. Avoid both, you never need to click very far though, so don't run across the room if you don't need to. Run and gun. You can really get loads of damage here, but if your health isn't full, attend to that first and do run across the room so you have lots of time to eat without any bombs under you.
- During the 'let the blood consume you', your best chance of avoiding it is to go in a straight line. This is harder to avoid than the darkness.
- During 'stare into the darkness', set yourself up in a straight line to Vanstrom and always have your camera facing his back, so you just have to tilt it down. You must be directly facing away: diagonals won't work as well, so be in a straight line. Standing near the desk works very well for avoiding this: it is not a hard attack to avoid and I think Vanstrom teleports before he does it.
- Minions: if you don't kill one immediately, switch to the other: your rapid attack will kill the first (hopefully) and then the second will die. If for some reason you're hitting badly on them, attract them to you, but make sure you kill them quickly as Vanstrom will continue to use other attacks once they are no longer going towards him.
Other: Teleport to your house, then use the portal to canifis to set your hub there before using the medallion. This ensures that you will not use a medallion charge to get back to Darkmeyer if you die (as you'll spawn in Canifis). You don't want to run out of medallion charges: Merch did, and kindly told me on Skype. Remember this, and be wary: if you're on your last charge, use it to Burgh de Rott.
Killing the boss will give you a great sense of acheivement and a very handsome amount of xp. You'll also get two spins on the squeal of fortune, so don't miss.
Ava's accumulator will attract stakes, but you'll lose it when you die. Similarly, Tokkul-Zo is useful until you realise that it'll cost 48k tokkul to get it back again. Just make lots of stakes: I used about a thousand and used pretty much that many as well over the course of my fights.
If you're having difficulty, ask someone who has done the fight for any tips. They'll doubtless be able to provide you with valuable information and (in the case of merch) also be able to calm you down.
TL;DR new stuff I wasn't told: use eagle eye and a ranged pot instead of protect prayers, use rapid style, get karils. And lots of other stuff, but that is the really important stuff that I didn't know.