Lancelot's main Artifice are her barriers. This version of Lancelot has barriers on her Ending Act that cannot be broken by normal damage. Her secondary Artifice is her absorb, which provides a steady stream of health back to her.
The support girl released to counter her is 4★ Shinohara Yua Ichie, a chopper with skills that can remove barriers. Alternatively, Reflect or MaxHP% dots work very well.
1) Standard Team
Same old, same old. Note that Ichie's 2AP deals damage before removal, making it less useful. A proper writeup will surface if anyone actually bothers to try it out, because there are far easier ways to do this.
2) Yua + Marie
Yua's 2AP barrier removal, Marie's barriers and buffs. Repeat until dead Lancelot.
1) I'm patient enough to wait 30 turns.
Spam Rui's barriers and Pollux's reflect. Over. and over. and over again. No memoirs are needed.
Profit, while mourning all the damn time you wasted.
1) All of them.
Various permutations of this exist, but it's generally agreed that the more MaxHP% dots you bring, the easier it is. New Year's Tsukasa can also contribute to damaging the boss. For example, if you only have Amaterasu Maya and NY Tsukasa, you will have to cast burn three times to deal 90% of her HP in damage, and one Tsukasa CX to make up the remainder. To cast her CX three times is no easy task.
However, if you bring Tsukuyomi, you'll only have to cast it twice to hit 90% of her HP. Add more to make it even easier, while bringing some stall tools along.
Beware, for her absorb could be a problem if you stall for too long, and she regenerates enough health to be alive after 90% of dots and NY Tsukasa's CX has landed. The solution? More dots. 120% or more of her HP, eat it with dots.
2) Gawain's poisoned dagger (so much for honourable knightly behaviour)
Lancelot or Match Girl Yachiyo plus Gawain. 5 turns, dead knight - Taunt and Poison every turn.
3) Just how much poison are you feeding Lancelot now?
Turn 1: Snow White AP down + Gawain Poison (10%)
Turn 2: Snow White Counterheal, AP Down, Gawain Poison (20%)
Turn 3: Snow White CX + Gawain Poison (45% overflows to 99,999 damage)
Turn 4: Gawain Poison (99,999)
Win!
4) Stalling and Burning
Match girl's provoke + Diaochan's heal every turn. Use a defensive memoir on Diaochan, as she must withstand Lancelot's AoE attacks.