Though me and you are thinking of a "last boss" Our ideas are different. My idea is that we have a boss that uses all attack types, but your idea deals more with the swarm. Though your idea is more indepth then mine I would say that this idea can be called "different" from yours considering it can be placed anywhere and can be reconsidered an alternate ending for rank missions OR a new SL arena or boss.

May I remind you that his attack style in red is centered towards evading all party members attacks until it decides to attack. Think of it like this:

A trojan focuses on one player at a time making it a easy target if you have 2 players or more to attack the trojan. Sounds easy correct? Alright.

The boss on the other hand is designed to avoid ALL party members making him harder to hit and harder to get to his weak spot. While he is weak to almost all elements of attack and status' he can deal intense damage considering he is "blunt in his attacks" making him more of a threat in combat then he is on defense.

True it could be an easy boss if you just spam a shivermist behind him but you'd need to get behind him in order for that to happen, but here is where we set the idea of "Damage changing phases" OR "Timed change phases" Either way you're still going to need to face all 4 phases in order to defeat the boss which can be time consuming, challenging and worth the wait due to the ideas that can be created if they make lout at the end of the match. (i.e. New armor, weapons, shields, accessories what ever it is you think should be the final outcome.)

Lastly there is one more idea which is HP. Each phase can have its own set amount of HP making him more heavy in HP in red phase then in all the others.

This is why vanaduke is classified as a fiend. Secondly its their job to do this so I don't think it would be much of a problem to start this off. Look at the Guild update, it took a while but its really fun to enjoy a good looking guild hall if everyone pays their way. Look at the rank missions from long ago, they didn't use to be there but now they are, what about the constant events that they uphold? Hacked series, hunter series, hallow series thunder and lightning series. All took time to do but they get here. But lets not forget the battle sprites that are about to be released soon they took a really long time to get here but they're still working on it. So instead of thinking its "Impossible" if someone can make a breadfish song with its own URL then we have much hope for the future that they can develop a boss like this.

Making them faster would be a challenge but much more of a pain in the ass so I say no to that. The tactic of giving him the ability to defend his backside by swinging behind him would be great! But what if thats another opening for a quick attack? For instance, when he attacks he does an over hand swing that smashes down into the ground but it has a medium cooldown untill you attack him once hence fending off second attacks? The blitz idea is a great idea where he would charge directly at charged enemies with a blitz gun in their loadout, but it would have to be on his red phase. On the yellow phase it wouldn't be smart but he could have a small flinch if you use a charge attack on him. On the phantom phase it would be useful to have a blitz. Secondly he would need a weak spot on the elemental phase and blitz could cover that or any piercing attack for that matter, the vines that he sprouts into the ground could easily snap with that idea so lets give the blitz some kind of chance in this fight, the vines could snap by like the 4th shot of a blitz charge, the first shot of a callihan, the first swipe of a Flourish line (5 stars) and the bomb could easily burst it in 2 bombs but the bomb would need to be chanced due to its wide spread attack.

Piercing needs to be effective at a point but the only way we could use blitz is if we nerf its attack ability, possibly a slight resistance to blitz making him more challenging. Also the boss takes normal damage towards all attack types but the one its resistant to so in my mind blitz would only do so much but slow the person down for a strong attack that a WMRH could easily do in a charged attack or quick swing.

One idea I do have is the ability to use defense up seeds to make him even more challenging giving the blitz some kind of use for struggling parties but it comes at a cost of slowing you down a lot. For instance what if the floor was covered in a seed that you could never really destroy? That would reduce the speed issue just focusing on his defense making him harder to hit but also more challenging to dodge (unless the arena was available around the corners where you could run normally) but something to work with.

Secondly we don't need another chaos line but instead what if instead of increasing damage we increased pet damage increase or their support? It could be a first... Also, the idea of a new armor set would be nice but I feel we should bring something new to the table like a new sword, gun, and bomb. One that would make everyone yearn to grab as a victory item! the almarian knights spear would be awesome to see, or a totally new idea of weapons like a sword that has a charge attack that's like a bomb instead of projectiles it does a blast radius on the ground. That'd be awesome to see in Lockdown matches when you're about to get swarmed it just blows everything up around you with a possible chance to cause any status effect that is randomly generated. Also we could use a bomb support shield, one that would provide elemental and shadow defense or normal defense with CTR medium.

As for your status resistance idea I say poop to it because it's just way too annoying and would render bombs useless in this battle and I want all weapons to be accessable in the battle and worth fighting with including the blitz gun (sadly) also, the idea to see a new strategy against an enemy that requires all weapons or almost every weapon to make him worth fighting if you've played the game to the brink of 100% completeness, and worth making a player want to work to get a good range of weapons for this battle.

Lastly i would like to see bombs used correctly in this game, not just spammed for the lulz (eventhough we all know shiv spammers are going to regret getting close to him in any phase due to the back attack idea) This would make great for teamwork plans or really good 1 man army people.