It looks like Vanya and especially Amor have changed the colosseum metagame... Pairing those two with Yatsufusa, another fire card (like Ashleigh) and another non fire card (to avoid dispersion of the buffs/heals) makes a quite strong deck. A speed focused deck is likely to not be able to outdamage Amor's healing at all after getting debuffed and then get butchered by Yatsufusa's counters.

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You can get the chest reward even if you leave the dungeon without killing the boss, just letting others know in case your HP is low and know you can't advance further.