Elisha logistics:
summon method: gem
abilities:
Headlong Rush, Range Up 1, SP Restore 1
skills:
Magic Blast (and Storm + Nova for extended range, also bypassing Aegis and React: Shadow Phantom*), Massacre, Convert, Summon Imp

Chronos logistics:
skills:
Whittle, Shock (this one is important for proc'ing enemy React: Jump Gate and the Barrier from React: Auto-Guard).
strengthen:
5 points in SP for an extra shock if needed, the rest in HP and Def to prevent some masters from one-hitting your Chronos.

Kamaitachi logistics:
skills:
Gale, Rushing Wind, Wind Burst (important to bypass React: Shadow Phantom*, also for hitting the master hiding behind an Aegis)

*the description for React: Shadow Phantom is a lie. It will trigger on any single-target damage skill regardless if it's physical or magical. Only AoE skills and probably status skills will not trigger it.

Turn 1. Use Convert for an extra SP. Headlong rush maximum in one axis, summon Chronos, Whittle enemy master. Most masters will start hiding/retreating after you end your turn.

Turn 2. Chronos: Whittle a second time. Elisha: Headlong rush in the other axis. If no enemy React skills are used, summon Kamaitachi toward the enemy master, then cast Convert again. If React: Dimensional Trap is used, use your skill action to summon an imp instead. (If you're facing Fan, she might use global summons to block Elisha's path. In this case, maneuver the best you can, and summon imps to physically get close to her.)

Turn 3a. If no enemy React skills are used (or if they used something insignificant like Block), then Whittle a third time. Next, Kamaitachi should get in just close enough for a max-range Gale, so she can subsequently move adjacent to the master again for chip damage. Move Elisha up for a max-range Magic Blast, then you should win next turn with Rushing Wind / Magic Blast.

Turn 3b. If the enemy master used React: Jump Gate, then Kamaitachi should use Gale for the first trigger (she can move immediately after, possibly adjacent for chip damage if you're lucky enough). Then, Chronos should use Shock to proc the second trigger (and then move back). Elisha should either use Convert or summon an Imp (if you went second). Wait for the last Jump Gate to expire next turn.

Turn 3c. If the enemy master used React: Auto-Guard, then Kamaitachi should first hit with Gale + move adjacent for chip damage. Then, Chronos should use Shock to rid the Barrier, followed up by a Magic Blast from Elisha. The second Barrier should expire next turn.

Turn 3d. If the enemy master used React: Petrify Counter, only use Chrono's shock to proc it. Summon an imp or use Convert if not enough SP. Repeat until the React expires. Kamaitachi's chip damage does not proc the React, so keep her moving next to the enemy master.

Turn 3e. If the enemy is a healer (e.g. Univa), you'll want to save your third Whittle in case she heals (use shock for minion combat and then move Chronos away from danger). Your win condition is to summon as many imps as you can while you wait for the React: Auto-Heal to expire. You'll need about 3 imps to completely defeat her (she usually doesn't use charm on familiars). The first imp can use Weaken on Univa while the React is still active. Kamaitachi's chip damage does not proc the React, so keep her moving adjacent to Univa every turn. After their attacks, try to position subsequent imps in a knight-move pattern away from eachother to dodge spin slash & air slash.

P.S. If you face Touka, it's doable, but it's gonna take a ♥♥♥♥ -ton- of imps and way more than 7 turns to complete. I suggest surrendering and starting a new match with a different opponent since Touka is not gonna be time efficient.

P.P.S I tried using Rimmon's "Remove Buffs" to attempt to remove enemy Reacts, but the skill misses. This might be another case of inaccurate description translation.

This is a detailed version of Edii's team, found in the discussion section. It involves cheesing out Thanatos in 4 turns by abusing Falim's "Requiem" and/or "Soul Beckon" ability to preserve SP, as well as her sacrifice summoning method to meet Thanatos' requirement.

Note that although this team is faster in terms of turns taken, it leaves no room for other creatures to leech EXP off of. It also involves a decent amount of clicking.

Setup
Falim: Necromancer
abilities: Sacrifice Sum, Requiem, Soul Beckon.
skills: Raise Undead, Sacrifice (the skill that immobilizes and silences enemy master).

Creatures:
Lailah,
Aphrodite (or another creature with high SP or low summon cost),
Tai-Sui (with Petrify),
Caladrius (with De-Stone),
Kelpie,
Thanatos.

Turn 1: Cast Raise Undead, Summon Lailah by sac'ing the familiar.
Turn 2: Summon Aphrodite by sac'ing Lailah. Then, summon Tai-Sui by sac'ing Aphrodite. Use Tai-Sui to cast Petrify on Falim.
Turn 3: Caladrius is auto-summoned because Falim is petrified; she should cast de-stone to cure Falim. Then, have Falim cast sacrifice on the creature that has fewer SP (Tai-Sui). Then, summon Kelpie by sac'ing the creature that has more SP (Caladrius). Have Kelpie tow Falim toward the enemy master.
Turn 4: If you did this right, you should have more than 22 SP even if you went first. Summon Thanatos by sac'ing Kelpie. Use Death's Decree on enemy master.

P.S. I don't think Thanatos will work on Touka, so concede and start with new enemy master if you face her.

P.P.S. I tried fiddling around with Nether Feast, but Thanatos isn't an undead type unless I use zombie poweder so it didn't work. I also tried fiddling with Lilith to sleep instead of petrify, but sometimes Falim is cured by herself the moment I end my turn, so it's too bad I couldn't use a cute devil for consistency.