I expect this guide needs some refinement to adjust to 'legendary' requirements. As a bio, it's somewhat hard to even get to the Baron, though once you're there, things start to become somewhat easier. Early on you are to get with destroy, reform and teammates, when you need them. I used points first into: vicious strike, break, integrity, adrenaline, reform (level 1 seems to be enough) and split attributes half str half vit. One good thing of reform - it heals for percentage of your own
life, so if your life is high you can act as a strong healer. Veradux depends on stats to heal, so you might use him as a damage dealer instead, and keep a disrupt+reform handy.
Twisted Experiment: The point when you have to watch over Veradux to not get him killed. Once the Experiment wrecks you with Withdrawl, set Veradux on phalanx and wreck the experiment with Disrupt, that will drop his healbuff, Otherwise he's a brute force.
The Warden - a lump of hitpoints with nasty attacks. Get Anesthetic and lower his ability to damage yourself. When he'd get 250 hp, go all offence and break him.
Frankie is a joke, set Veradux on relentless, break Frankie so he won't 'guard' his ally, then whack the doctor, after that just heal and wait for Frankie to expire - he pays life for his strikes.
ZPCI Elite is a weakling if alone, get 2*disrupt, you might already put 2nd point in there, you'll need it anyway, then disrupt both his healing buff and his enraged buff, or break.
Baron - get 4*disrupt (lvl2 required), put both your kids to defence, and then just disrupt-disrupt-disrupt the Baron, he will whack you with Electrical Storm once per 2 rounds, which can be easily healed, and he will just suffer. Continue until he'd drop. I'd advise replaying this fight until the Baron will drop the sword, there's a nice sword :)
Clemons - by this point you might decide to go all poison, though Clemons himself is resistant to poison damage. Anyway, once you get Infect AND Crystallize level 2 at least, you're set against any single opponent. You crystallize, leading strike (dealing no damage but restoring focus), then when your enemy has 1 on crystallize left, you Infect. The DoT of Infect will deal damage. Rinse and repeat :) However, this is a loooong battle tactics.
Gregor - oh well, he heals? Pfft. He's no match for crystallize/infect tactics. Note also, with this you don't need much speed - you will want to have your allies do some damage to help you, before you crystallize your enemy into an invulnerable rock.
The Hydra - side heads can be killed by poison (Envenom is preferrable if you strive for achievements) with a single shot, then prepare to a long long battle. The Hydra itself is highly resistant to poison, as well as to shadow, lightning and ice. Anyway, if it's down to one head, you know what to do.
The Council - edited: they are ComboX'able, although you might just not have enough power to one-shot one of them with just your own efforts. You are to battle them at level 20, and you will get level 21 just after this battle, so you're not yet to use all of those nice equipment you may have collected and may shop in Z5. For the maxed ComboX you are either lose Crystallize 3/3, which should be used in this battle, or AE, or Integrity, or Shadow Blend. You have 24 skillpoints, and you MUST use 3/3 crystallize, since the recuperation will be long, and you have to recover focus, wait for Withdrawal to cooldown, while making sure you can charge your entire combo and not lose any of your teammates. In this battle you should use Felicity over Roald, because she can silence and break, and has formidable damage, and can use level 21 equipment, and has enough speed for you to move first without shadow blend. You are not to move twice in a row, to not get one-shot if you fail to one-shot the second council.
The build is as follows: Crystallize 3/3, Withdrawal 4/4, Toxicant 1/1, Shadow Blend x/4, Integrity 2, Agile Exposure y/3. This will use 4+8+2+2+1+2+x+y points, which is 19+x+y, so you are to either have AE 1 and SB 4, or other combinations. The complete equation directed to maximize damage output takes your stats as base, but doesn't include any 'misses', AI reactions (say Felicity might silence the undefeated council in the right time, so even if you fail to slay it, you still have time to assault it with one more ComboX), enemy moves, etc etc. so stick with this.
However, with legendary runs you're unable to train, get more exp, money and loot, so you'll have to plan ahead at least to Z3, where things start to rock in favor of poison build, as well as in favor to speed build. I recommend the following build order:
Level 1 - respec ASAP, you'll get missing attribute points. Get Break and Reform (5/5 spent).
Level 5 (The Warden) - get Anesthetic, Disrupt, level 2 Reform and a point to spend somewhere, hum. Integrity is an obvious way, although you might not need this.
Level 6 - get Toxicant form. +40% damage, +20% heal self in 4 turns. Prepare for a long battle against the girl.
Level 8 - get Endurance (a prerequisite to Crystallize) and second level of Disrupt.
Level 10-12 - 3 points in Crystallize. The point at level 9 might be spent towards Corrosion (into Slash, although you aren't yet to use it). Some of you might get along with 2/3 Crystallize, since it has a cooldown of 4, and you can have 2 of them at once in your skill bar.
Level 15 - get Corrosion+Envenom+Acidic Blood. You're a poison bio for some time, but you're still a healer - there was no re-spec.
Level 18 - well, you might re-spec to speed bio right now. Hope you'll have Emerald Death to fight with. Still, if you're to re-spec, the Unholy Avenger is the sword to use, for it's the only one with shadow piercing available, but having ED in your possession won't hurt. :)