My suggestion is to make a new custom item that deals extra damage specifically to EMC's Custom Mobs, similar to how a shiny arrow lets marlix bow deals extra damage, but regular damage to standard mobs Edit: Possibly a craftable combined with dragon stones/fragments?
https://getyarn.io/yarn-clip/79724af0-5238-4b17-8107-4c56cf52f043 Something like this... How very appropriate! Smp8 leads the way