You simply need pd, a lazu and gunslinger, Cosmo and walla, 6th is optional but I have lava but you don't really need her for this. Gunslinger will be the killer, you need her to have sunder as high as you can level it and brawlers favor, took his health away in two seconds literally. Walla, cosmo, and pd should have empower, you need walla for his slow down on boss, pd so it doesn't take him forever to get hits on heroes to activate skill so put him alone so he can stack on them, then Cosmo to stop and freeze the mechtessa's from running into your base, a perfect score right here, test it out. Note my heroes are all double evolved and lvl 200 in this advice I have given, some have breakthrough aswell, results will vary upon builds on your heroes. Good luck to those who are having a hard time killing him!