We have plenty of empirical experience suggesting that 5*s that don't hurt your opponent harder than 5 (Haxx, Essie, Eryton, Nistarok, Miss Lulabee) or 6 unprotected damage (Lulabee, Serena, Selena, Stella, Miss Stella, Neil, Dwan, Fabio, Jimmy) don't get used in ELO.
The minimum for a playable ELO 5* seems to be: 5 damage with pill manipulation or poison, 6 damage protected by SOA or protection: damage (Lena, Maazk, Hawk) or 7 damage (several playable characters in this group such as Beeboy, Bodenpower, Havok, Sledg, Michael, Lizabth and Dorian).
7 seems to be a tipping point number. The typical 3* character played in ELO has 4 base damage- there are no 7/5 3*s, there are only 6/5 or worse 3*s. This means that the average character can only hit to 6. Hitting with 7 damage means your opponent is extremely unlikely to make up the difference by using a 3* character. The simplest winning strategy in ELO boils down to hitting with a 7+ damage character while negate your opponent's own characters that hits for 7+.