Tristan's main Artifice is her casual deletion of one girl at the start of the battle and her continued casual deletion of a girl with every CX she does. Her Opening Act deals approximately 99,999 damage times four to a flower-type girl, thus making it entirely unsurvivable. Her CX does approximately 10% of that, about 9,999 damage per hit, four times. That is barely survivable for a flower tank and will oneshot anything that isn't.
Her secondary artifices are 1) her ability to stack her ACT boost up to 90%, 2) her chance of confusion, and 3) her removal of all positive effects from enemies. This disables Diaochan Ichie's regeneration.
Tristan is also immune to MaxHP% DoT.
Nightingale Rui is the support girl used to counter her, as she possesses a Brilliance Down attack to prevent Tristan from using her CX, as well as a revival CX.
It is also possible to nuke Maya down before she defeats you.
The "Normal Reflect Damage 10%" memoir, which can be acquired from the Seiran event, when placed on a girl with elemental disadvantage to Tristan + one innate fortitude or a neutral girl with two innate fortitudes (e.g. Stage Girl Karen, Heracles Maya) can shave off about 25% of Tristan's HP via the opening act as long as they are targeted by it. Vampire Karen, who has two innate fortitudes and elemental disadvantage, can shave off up to half if lucky.
1) Standard Full Team
Remember to keep the brilliance down effect on Tristan as much as possible. Include two flower tanks and two attackers as well.
Otherwise, play as normal.
2) ...or maybe they don't need to.
A US21 flower tank or a mid-high US backrow HP that stands in front of Nightingale. They get removed by the opening act, and thereafter Nightingale spams her counterheal and brilliance down.
1) One foul at the wrong time and she's out.
This strategy requires 110+% HP boost on Awakened Mahiru, and over 125% to be considered reliable, as Tristan's CX hits are capable of critting.
The confusion that Tristan has also throws a wrench into the strategy as failing to heal after taking Tristan's CX will be disastrous.
1) Prince Nana + 1
The core principle is simple: by using Prince Nana's heal immediately after Tristan uses her climax, enable the tank to hang on long enough to take a second climax from Tristan. This requires at minimum approximately 68% extra HP from unit skills. Two flower tanks, Prince, one other attacker, and one girl to take Tristan's opening act.
2) Seisho Flower Tank Stacking
Mona Lisa Hikari and Momo Nana are both Flower Seisho tanks. Now, conveniently, Awakened Mahiru and Stage Girl Karen's unit skills affect them both. The concept here is simple - load up on a truly incredible amount of HP from unit skills and slowly bring down Tristan's health while incessantly healing your tanks.
1) Have the whole team cosplay a Pyro Regisvine.
Line up a bunch of flower attackers with high multipliers and some unit skill support to take the initial blow. Just kill her faster than she kills you.
2) Tristan in Wonderland... or not?
Everything lined up for Dorothy to take the cake. Whoever your sacrifice is, put a Frontier 1st. Anniversary Memoir to increase Dorothy's damage. The main failure state of this run will be Tristan confusing and cancelling Dorothy's CX, but it's not too common. Remember to cast Dorothy's ACT/DEX self buff before her CX for maximum damage output. It's important to have Dorothy backmost for this strategy to work.
Replace Dorothy with Nobunaga Hikari for better results.
Notice that Tristan's opening act does not remove positive effects. A memoir giving reflect at start has been added. The result? Removing some of Tristan's HP before the battle even begins.
For this Tristan, Vampire Karen is the most viable girl, as she possesses two innate fortitudes and her element is weak to Tristan's. With a 10% Normal Reflect memoir (MLB), she can remove up to half of Tristan's HP immediately.
With a 5% reflect memoir (nonMLB), or less than two innate fortitudes, the effect is much less pronounced.