The intent was / is that using Dragon Slayer in your line up should make starting energy plays viable or be a way to win the energy race against a drain-focused team with a hero like Cyclops Shaman behind her.
So: A Dragon Slayer lineup vs a Bone Lineup should be somewhat shielded against the initial drain from Bone Dragon. She is wielding the standard even if it hasn't been placed yet. Her placing it is done just so that she is freed up to use her club.
Alternatively, she should help to shield against Void Wyvern for a bit.
A player facing a Dragon Slayer lineup could focus on heroes that don't bother with energy drain for other benefits or they could double down with Bone and Void. She was meant to encourage diversity in lineups and to have a pathway for one of three coliseum lineups to use starting energy or to introduce some degree of rock paper scissors to fight pits as people rotate around countering different lineups.