ATM Vanaduke is massively difficult.
Probably not impossible but still is.
Having to resort to splitting 4 player parties into two 2player parties and then having to invite random people just to have them save you and die on the spot is not a good way to make a boss.
At the moment, average CE spent is astronomical.
Over in the 1000's of CE wasted on this boss.
And all for what?
3-5 Almire Tokens.
Even worse, people are relying on a bug which opens supposedly unopened doors to gain access to the sweet, sweet infinitely respawning hearts behind the gates players have abandoned, and a bug during the shooting stage, which has a player intentionally kill himself in front of the head, keeping it in place until it dies.
1. First off, he has SEVEN STAGES. No reasonable boss has that many forms. Although there are only 2 distinct patterns, they get harder each time. This is especially bad because he has aton of HP.
2. A MASSIVE ramp in his HP when 3+ players are in one party. Any sane person would have to resort to the above scenario I mentioned. He also has an insane amount of HP Per Phase.
3. This is more of a general issue with the flames itself. They have erratic paths and they're unpredictable. When I thought I was safe, they would always make a curve in the middle of a straight path. What're they, homing? Same deal with Flame Rovers, but they aren't as bad.
4. Water cannot go through bullets. This could just be a general bullet issue itself but it's very promenant in Vanaduke's last stage.
5. Fireballs should not revolve around him. This gives him extra range when he dashes. I am consistently sideswiped because I can't seem to move fast enough. This also makes close range weapons like swords and bombs increasingly dangerous when you go futher into the boss. If fireballs are needed, then make them simply obsticles and restrict your path of movement, or make them revolve around the center of the stage, not around Vanaduke.
6. Glitches that may occur during phase switches in which he may actually get STUCK. He has a spot where he goes to change phases, and he may just sit there and not do anything, halting the boss battle completely. There are reports that throwing water at him while he changes phases may get him stuck.
7. Damage in general is wonky. Kekelgis has posted a thread on how some equipment barely made any changes.
http://forums.spiralknights.com/node/1556
8. Slag Guards do not drop any sort pickups or health. Only Slag Walkers do, and they stop spawning on the final phase. What's worse is that they infinitely spawn. They're not too hard by all means, but when you have a huge boss destroying you, you may need some health to keep living. You can't expect us to keep 40 bars for seven phases and expect us to not heal.
This is what I believe is the issue.
Some suggestions I'd have for this;
1. Reduce the whole damage in tier 3 altogether. With the recent changes, the damage you take is pretty insane. I used to take at most 9 hearts at depth 28, on before the whole rebalancing updates, and I wasn't even with my current 5star stuff. Now I'm taking 9 hearts at depth 19. It's quite ridiculous.
2. Reduce the HP he gains if you have more party members. A bit too extreme of a change.
3. Either severely reduce the amount of bullets on the odd-numbered stages (Shooting stages), or severely reduce their damage. It's literally impossible to get past Phase 6 without getting harmed several times. Not even the most amazing bullet hell player with an awesome internet connection would be able to dodge enough to survive AND hit him with precise waterball throws, AND deal damage to him.
4. Make it so the bullets don't block the waterballs, or only one kind of them (Red bullets or orange bullets) blocks. On Phase 6, it's really difficult to get a good timing to hit him in the middle of all those bullets.
5. Either decrease the amount of Seals needed for the items on the shop, or increase the amount of Seals he drops. I mean, come on - An average party would need several revives (and by several I mean going over 2000 total cost of energy only on revives) and appropiate gear to do it, and it takes quite long to defeat him, and even more to get past Firestorm Citadel. Even an experienced party would have difficulty. Only getting 3 to 5 Seals is not rewarding at all, considering premade equipment costs 50 Seals, and a Dragon Scale costs 15. Nothing else in the shop is really worth all the effort either.
6. Perhaps give some hearts/health (if not a complete heal, even) at the defeat of each stage? Getting past a single stage is already really difficult, then you start a new one with no health to survive more than a single hit, and everyone else on your party is dead. I mean, come on.
7. The fireballs that circle him should circle at slower speed - Ranged damage won't deal much on him, and short range gets very risky because of the fireballs. If you're hitting him from the sides, sometimes you can't even see the fireball coming at you. Another option for this would be what Hiro said - Making it so it circles a certain point in the stage instead of circling Vanaduke itself. Maybe it could even be just a general static obstacle, such as spikes or fire traps?
This also applies to the moving balls that release bullets on his shooting stages - They go way too fast, and sometimes you can't even see them coming. When you notice them coming, it's already too late to evade them, because of their massive speed. I've got hit by them countless times because of this.
I think that's about it. Obviously, not ALL of those should be implemented together, though most can work well together and would probably balance him fairly.