Lancelot's main Artifice are her barriers. Estimated to be around 30,000 and cast each turn (and can stack). While one turn's worth of barriers can be pierced, once stacked they are insurmountable walls. Her secondary Artifice is her absorb , which is only present on one form of Lancelot. The Lancelot without boasts a higher damage output, however.
The support girl released to counter her is 4★ Shuten-doji Fumi, a chopper with skills that can remove barriers. However, damage over time ignores barriers. Those without Shuten Doji will likely enlist the support of MaxHP-based DoT.
Everyone that can remove her barriers can also remove her absorb.
1) Fumi's not drunk, is she?
Just the way Ateam intended. Whenever Shuten-doji uses her 3AP or CX, the barriers are removed for the rest of the turn, so you must take advantage of that opening to smash through and deal damage to Lancelot. Of course, this means that drawing and using Shuten's skills at the right time is important. Leave your hold slot open on the turn of Shuten's CX, so if you draw her CX and Shuten's 3AP you can hold Shuten's 3AP for the next turn. Using other girls with barrier removal helps too.
Shuten-doji and Cinderella Mahiru 2girl can clear the boss, but it's a little close.
2) Bootleg ponzu works too.
Using Jekyll and Momotaro's cleanse, combined with some stalling and midshitting, you can achieve victory. Jekyll's US is very important here, and should be 14+. You should aim to get off two Momotaro CXes. Remember to keep the hold slot open for Jekyll's cleanse - drawing both on Momotaro's CX turn is quite a waste if you can't hold it. Near end-battle, using Jekyll's cleanse followed by Nurse's 3AP+1AP should be the norm.
If you have Cinderella Mahiru in place of Belle Shiori, the battle is substantially easier and has lower US requirements.
1) Open the floodgates.
Various permutations of this exist, but it's generally agreed that the more MaxHP% dots you bring, the easier it is. Nurse Mei Fan can also contribute to damaging the boss. For example, if you only have Amaterasu Maya and Nurse, you will have to cast burn three times to deal 90% of her HP in damage, and one Nurse 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 on the Gawain ver. Lancelot, 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 a Nurse CX has landed.
2) Gawain!
Gawain + Cinderella Mahiru. Taunt + Poison every turn - five turns, dead knight.
1) Nurses creating work for themselves
Needless to say, you'll need high unit skill levels on everyone save Momotaro. Chip off Lancelot's HP with Momotaro's nuke and Jekyll's cleanse, if available. The objective is to stack Cheer Tsukasa's dexterity buff to achieve +60% dexterity and nuke Lancelot with Nurse Mei Fan. If she has some health remaining, Jekyll and Tsukasa can clean that up. Time Momotaro's climax to allow your final act to deal damage, but it's not compulsory.