The second Chrominius must be injured to less health than Twlight Clutch-Sister (1465 HP). Start a setup fight with Chrominius, a healing pet (Bleat around 195 healing), & a lower health pet. Void Gate injures Chrominius & swaps to healer. Bleat, Chrominius heals. Abandon setup fight. Put healthy Chrominius in the first slot, TCS in 2nd, injured in the last. This ensures that Twilight Clutch-Sister is used in Turn 2. Non-power fawns work well for the Bleat.
At least one of the pets for this strategy require the use of a specific breed.
Dragon team with older expansion pets if you don't have the Nightmare pet from Legion.
Read the note above. Chrominius, healer, other pet with less health. H/H, H/S, & S/S fawns work well as healer pets for this. Power breeds may over heal.
Pass, Chrominius is injured
healing pet is swapped in
Bleat, heal Chrominius
Check Chrominius health. It should be near 1450 and <1465.
Set up real fight with injured Chromimius in slot 3.
Pass, Void Gate swaps in Twight Clutch-Sister
Swap back to slot 1 Chrominius. Since Void Gate injured him, he will be the lower current health.
Pass, slot 1 Chrominius dies
Tail Sweep, here's the RNG part, TCS must survive this round,
Pass, TCS dies, slot 3 Chrominius swapped in
This strategy was not created by me. Supposedly NoGulpFrog developed it.
How it works: The TCS is used to protect the slot 1 Chrominius & damage with Twilight Meteorite. The Chrominius has a high health pool. It does heavy damage with Bite, and Surge of Power, but needs the block from TCS.
I haven't tested this with the P/S, & S/B breed Twight Clutch Sister. P/S has less health and will need a different sequence to get injured Chrominius health right, and will be more RNG dependent. P/S may not even work at all.