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 snow-type girl, thus making it entirely unsurvivable. Her CX does approximately 10% of that, about 9,999 damage per hit, four times.
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. While her initial damage isn't high, it quickly intensifies as her ACT Up begins to stack. The confusion application serves to throw a wrench in the stability of teams that run on a knife's edge, while the positive effect removal prevents cheese strategies.
Tristan is also immune to MaxHP% DoT.
Hellsing Michiru is the support girl used to counter her, as she possesses a revival CX with chop. Her glorious card art is now shown here for your adoration and veneration. Continue reading after you have been sufficiently bedazzled and recited "Michiru is best girl" five times.
Or don't, I can't arrest you for shit taste.
Focusing on nuking Tristan down is of utmost importance. To that end, Cheerleader Ichie, Ryugu Dancer Ichie, and other Snow ST girls are very useful. Nightingale Rui, used against Wind Tristan, remains usable here, as her brilliance down effect is great for suppressing Tristan, who isn't much without her CX.
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.
1) A frozen rose, to cut or to bloom?
The most offensive-oriented version of this strategy uses one high Snow defensive US, and leaves the rest of the slots to attackers. The principle of this tactic is as follows:
Hold Dracula Claudine's 3AP and charge the Snow attacker in third position on turns one and two. On turn three, after the unit in second position is defeated by Tristan's CX, use Dracula's 3AP followed by the third position unit's CX, which should heal her enough to last until turn 5. Each girl must be defeated by Tristan's CX and not her normal moves.
2) I don't have time for this, I really don't.
Who needs absorb or survival? Just... nuke her. The aim is to use Stage Girl Junna's climax followed by two snow 394% climaxes to remove a majority of Tristan's HP, and clean up the remainder with Ichie. While the reliability is higher than the Dracula strategy as you give confuse less chances, it's still not very high. It is, however, extremely fast.
3) The best Tristan team. Really.
It may come as a surprise to you that the best team for this boss uses neither revivers nor dim. This team has a perfect balance of survivability and damage, but requires US16 Awakened Nana and US21 Seisho Nana. BS Junna may be replaced by Musashi Nana or any Seisho Snow who stands in that position.
Awakened Nana dies to turn 3 Tristan CX, Akira on turn 4, and Junna on turn 5 CX. Dracula finishes the boss off. Use Dracula's 3AP absorb to keep Junna's health topped up enough so she can last till turn 5. Ideally, Junna's climax followed by Akira on turn 4. But if that doesn't happen, it's still alright.
Estimated Success Rate: 90-95%. Time Taken: 2 minutes.
1) Lancelot goes bonkbonkbonkbonkbonk
Monte can be replaced by any other passive taunt; she is used to give Nightingale's damage a slight boost. This strategy involves carefully juggling Lancelot's taunt to protect Nightingale, Nightingale's dim and counterheal, and Lancelot's attacks. Key to this strategy is Lancelot's CX which prevents Tristan from using her third act (front 3 confuse) for 3 turns. During these 3 turns, there is no need to taunt, and alternating dim/counterheal with Lancelot's 3AP+1AP becomes standard. It is therefore important to wait until Lancelot is not confused to activate her CX, because failing to use it will be bad. There is ample time in turns to wait out the confuse with Rui's dims and counterheals. Use a -15% flower damage memoir from the PVP shop (either one is acceptable) and Seisho Anniversary memoirs on the other two to boost Lancelot's damage.
If you revive Monte Cristo, you're a dumdum and the run will fail.
2) Rui's Revival Challenge
Amusingly enough, Nightingale Rui is a better CR support for Flower Tristan, because she has a Brilliance Down move, unlike Hellsing. This team stalls it out and revives Lancelot quite a few times, while slowly chipping away at Tristan's health.
3) What's better than one reviver?
Two, of course. Maniacally reviving girls is nice. Judicious delaying of revival to allow Hellsing to accumulate some damage and boost her enmity damage is quite possible.