im around the lvl 37 - 42 but the Pokemon boss battles are always higher :(
In Ultra, Nanu's team are at level 43. They all have weaknesses to Fairy, and all but Sableye are also weak to Bug and Fighting. It's pretty doable if you're at least level 38, 5 levels down, if you have moves or Pokémon of these types. Bear in mind that Sableye has Shadow Ball and Power Gem, Krokorok has Earthquake, and Persian has Power Gem. Sometimes half the fun is pulling off the win despite being critically underlevelled or handicapped in some other way, so it depends on how spicy you like it.