Gawain's main Artifice is her infliction of copious amounts of poison. Because this stacks, left unchecked you will take more and more damage from poison. If left unchecked, your team will take 105% of their HP in total poison damage by the end of turn 6. She also has attacks that amplify the damage she deals to enemies that are poisoned.
Gawain is immune to MaxHP DoTs, and all crowd control effects except Freeze. When frozen she gains a 5-turn immunity to being frozen.
Poison Immunity. This is most commonly found in the form of 3★ Easter Bunny Lalafin, though Heracles Michiru also has it. Lalafin's total lack of defensive stats means that she will be down quickly if attacked, so make sure to place her at third position or farther behind to ensure she won't be attacked by Gawain's front 2 attacks. Using the 2x Evade memoir (4★ Confidants on a Date) on her is recommended, so she can avoid taking damage from Gawain's CX too.
1) Standard Team
There are dozens of permutations to this team, and that number will only grow as more snow girls are added to the game. Essentially, all this team requires is one to two 4★ snow tanks, Easter Lalafin, and then two snow attackers. Easter must be in third position or behind, though.
4★ Match Girl Yachiyo is very useful here, as her counterheal which goes with her taunt is extremely effective against Gawain's multihit attacks. Nevertheless, after casting poison immunity Gawain is quite the pushover. Any coherent snow team should do the trick.
2) Dracula Claudine Solo
Dracula Claudine and any Poison/DoT removal+resistance must be the two backmost girls. The front 3 can be any manner of unit skill/stat stick walls to keep the last two alive and to boost their damage/health. Castaway Kaoruko is a superior, though less accessible, replacement.
1) Let knights eat cake, just remember to keep the poison off the cake.
With no stops to get in the way this is even more trivial. Cast Marie's barrier and poison/DoT resistance on the first turn, subsequently Marie moves only. Complete the bonking in five turns. Castaway Kaoruko works even better here.
2) This cake was most definitely not a lie.
A knight boss, finished off in three turns? Right here. Nana may be replaced with a US19+ midrow offensive that stands in front of Marie, though a three-turn finish will be rare in that case.