Team Cas' plan is as comprehensive as it is convoluted. I'll be going into further detail here but... well. I made a cheatsheet. I'm very sorry.

Moving to the matter at hand: Sans, Phoenix, and Blake are hesitant to kill Cas, and not just because she's connected to the Wellspring. Phoenix will say they have to try and talk her down, if not for Casariel's sake, then for Rainer, who has watched her best friend betray her over and over for the past six years.

So, with that out of the way. The Plan is divided into three stages:

• STAGE 1: Words, Not Deeds: Believing in the power of words to bring Casariel back from the brink, Blake and Phoenix will be attempting to negotiate with Casariel, and end this conflict before it crosses the point of no return. While this is the primary stage they will be doing talking in, they will attempt to convince Casariel to stand down at multiple points through this plan.
  • Phoenix will be taking the lead during this stage, with Blake supporting him, and Sans on standby.
  • Phoenix will be trying to leverage Rainer as Casariel's closest connection. When she inevitably claims she feels nothing for Rainer, Phoenix will raise a single OBJECTION! If Casariel feels nothing for Rainer after all this time... why is she still wearing that bracelet? He'll be using this to leverage the idea that Casariel was starting to get cold feet about what she was expecting to do, and what that would do to the people she cared about most - she had to commit to doing something drastic and irreversible to commit herself to her ill-fated course. He will be invoking Empathic Duty as well as Insight, the first to try and stabilize Cas' emotions (and make seeing reason easier, ideally) and the second to find the chinks in Casariel's facade and pry them open - as well as try and catch Casariel being sneaky if she tries a 'fake surrender'.
  • Blake, meanwhile, will be leveraging family, and the bonds that Casariel has made in order to beg her to see reason. To that end, she's using her Silver Tongue and, on a tip from Ferran, recruited the temporary assistance of Adriel Fontaine in the hopes that he can make 'Aunt Cas' stop her reckless plan, Blake will be encouraging him to show his Lucky Ticket, to show just how much Auntie means to him..
  • Sans won't be helping talk down Casariel, instead, he'll be setting up for the next part of the plan. He'll have used Digital Phantom to try and conceal the true nature of the Chance Lance from Casariel (making it look like an ordinary lance, instead of the powerful magical weapon it is) and while Phoenix and Blake are talking, he'll be using his Tarantula Ring as well as Misty Step to try and flank her without being spotted. If Casariel either spots Sans out or manages to divine just what Phoenix is wielding, they're going to skip straight to Stage 3, but otherwise...
• STAGE 2: Threading The Needle: If Casariel refuses to stand down, and Sans can get himself in position, the three of them are going to attempt to restrain Casariel, so that Phoenix has a clear shot to use the Chance Lance's one-time power. The goal? Splitting Casariel from the Wellspring.
  • Sans takes the lead on this one, with Phoenix as a support role and Blake on standby.
  • Sans will be using a variety of methods in other to try and pin Casariel down - he will most likely be using his Tarantula Ring to stick to the ceiling, and therefore restrain her via clever use of Levitate. If that doesn't work, he will also be using the Web spell, as well as the Spider's Thread that Maya got from Bryan (with a Y) for some heavy duty lockdown.
  • Assuming any of those stick for an extended period, Phoenix is going to take his shot - if nothing holds Casariel down for very long, Phoenix will hold off on using the Lance - after all, he only gets one, well, chance. But assuming this is where he has to take his shot, he'll be praying to Istus to split Casariel from the Wellspring - and, hopefully, make this entire affair much, much easier. The Chance Lance is the fulcrum of their plan, and so they've invested a fair bit into it. Phoenix will have use of Sayori's Once-Per-Mission-Log Book of Advantage so that his aim is true.
  • Blake won't be directly helping in the restraining of Casariel, as her job is to lead the engagement on her if she breaks free, and the trio is forced to fight. Which, speaking of...
• STAGE 3: All Hell Breaks Loose: The final stage of the plan is technically the simplest but has the most conditions to it - it's likely inevitable that a direct confrontation with Casariel is going to come down to a fight.
  • Blake, with her combat experience, is leading the charge on this last stage of the plan. With Phoenix (who is hardy but inexperienced) on defensive duties and Sans (the One HP Wonder) on Support.
  • Blake is a natural fighter, which is something neither Phoenix or Sans can claim in their resume. In addition to that, she is very hard to hit, with Evasion and Shadow Clones both coming in handy here to keep Casariel from being able to lock onto her for very long. She will be the one primarily trying to wear Casariel down on the offensive.
  • Phoenix, meanwhile, will be playing the role he was born to play: the Tank. Defense and Disruption are in his repertoire, as he is able to momentarily disrupt Casariel's spellcasting through either Objecting via Vicious Mockery (hopefully causing her to flinch and lose the spell), or by telling her to 'Hold' It with a Command. He'll also be slinging out Shield of Faith, particularly on Sans (though he won't hesitate to use it on Blake if the need arises) to protect them from any spells that do come through. Worst comes to worse, he'll try and intercept the spell with his shield, a move he'd been practicing recently. In addition to all that, as part of Minato's plan for Team Wellspring Defense, Phoenix took part in Minato's Ceremony, allowing him to communicate with Minato should things go horribly awry.
  • Sans will of course be trying to play the role of hard crowd control, leaning heavily on Web and Technovirus to keep Casariel disrupted, as well as throwing out a Haste to buff the trio in combat. If Phoenix has not yet had a chance to use the Chance Lance, Sans will be prioritizing trying to restrain her once she's taken some combat wear and tear, in hopes that gives Phoenix a wider window. He'll also be making judicious use of Misty Step to keep as far out of the line of fire as possible.
  • While not present for the confrontation itself, Maya will be using Summoner's Golem in order to provide a fair amount of resistance and toughness to the trio while they fight.

The three of them will be prioritizing using the Lance's magic to try and split Casariel from the Wellspring, and then detaining her if she refuses to surrender. If the Lance fails to split Casariel from the Wellspring, and she refuses to stand down, Sans is prepared to use the rest of his caster abilities to kill Casariel. But only when all other options are exhausted, and it's down to destroying Casariel - and all of Faerun's magic in the process - or letting the Hunger through.