Currently, there are two ways to fight Vanaduke:
1. Someone brings a shivermist, everyone else bring Blitz Needle and the entire fight is severely easier than any given fight with any other boss.
2. You do anything else and the fight is kinda unfair and not actually very fun due to broken elements about him.
Option 1 will let you beat Vanaduke in not more than five minutes, option 2 will take at least ten, maybe more like half an hour if you beat him at all.
Firstly, let's fix the unfair things that make legit fights unfair and tedious:
Before respawning, fireballs should "fade in" through some kind of blinking animation, warning players to get out of the way.
As it is, they just pop in out of nowhere and set you on fire. After watering Vanaduke's balls, they appear at a set time that is fairly long; it is unreasonable to expect a human being to keep track of the exact time each and every ball will reappear. Shiver + Needle is a player favourite not just because of the short time it allows you to kill Vanaduke, but because nobody wants to deal with watering the fire balls and getting close and personal as the risk is simply not worth it.
Vanaduke's dash can easily become unavoidable during phases 3 and 5, as the fireballs surrounding him will catch the players and often trap them.
If Vanaduke dashes into you while you're anywhere near a corner, you're pretty much dead. Instead of fixing the fireballs to his body, make them hover after Vanaduke in his wake when he dashes, "catching up" to him in time. This adds a bit of a risk-reward element to baiting Vana's dashes as it will leave him open for a few hits while the balls catch up.
Shadowfire, when first spawned, should give a five-second "graze" period before they actually become capable of harming and blocking players.
If they were simpy unable to spawn inside players, that'd make players just stand still next to Vanaduke charging their blitzes. This one makes the shadowfire less unfair.
Rewatering the mask should just refresh its downtime.
Or at least not add another counter. While there is an element of fun in timing watering the mask so that you can keep him constantly down, the penalty for throwing too early is arbitrary and feels buggy. It's also really nerfing the fight up for shard bombs in particular.
Shard bomb shards should be able to spawn inside Vanaduke instead of just spawning right outside his range or disappearing.
Not directly related to Vanaduke, but can we at least be able to even hit the magical 3-shard hit limit you implemented?
Just in case you thought this was going to be all nerfs:
Vanaduke should just smash the ground in anger to spawn firetiles when he's shivered.
Shivermisting Vana is still a valid strategy this way, as it will make the mace and dashes much easier to avoid, but players will still have to deal with something.
"Currently, there are two ways to fight Vanaduke:
1. Someone brings a shivermist, everyone else bring Blitz Needle and the entire fight is severely easier than any given fight with any other boss.
2. You do anything else and the fight is kinda unfair and not actually very fun due to broken elements about him."
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We can stop reading right there... because your basis of needing a change to Vanaduke is absolutely wrong.
I have 1 to 2 bar ping connecting from Hong Kong, and yet I have beaten FSC + Vanaduke solo wearing Proto helm, armor, shield, no heart trinkets, using only Glacius and Polaris.
If using 5* gears, I did many FSC runs using 0 pills 0 remedies from start to end.
Many other players have done the same, or feats even more crazy (e.g. no vita pod as well), long before I did that.
If you search my name, you will find me starting a thread in the New Recruits forum, asking about how to deal with Vanaduke. Many players chimed in and gave me advices and posted videos.
Learn from that and get better, instead of asking difficulty to be turned down for you.
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EDIT:
Here, I dug up my old thread for you.
http://forums.spiralknights.com/en/node/53818