That challange already finished at 15th Nov 2020.
This is why I leveled 2x Grimm. For this trial of Survival and also Trial of Strengh (all Red), stage 3 last bosses are Azlar - Elena - Nashgar.
But now, there are other options like: Frank (blue barbarian), Jott (blue barbarian), Sapphire (blue Rogue), Frosty (blue healer).
It is always the same for all members, not random… so it should be Red trainer.