Launch the game in team mode, extreme, normal, full game. The element choice order is Water 1, Fire 1, Dark 1, Water 2, Fire 2, Dark 2, Water 3, Fire 3, Dark 3, the element after this does not matter anymore. Start with 6 vapour and then 6 poison, build 2 infernal near the portal and manual control them to hit the boss when spawning, 2 level 2 infernal should be able to do the job nicely. Afterward, just keep spaming vapour & poison. You may turn one of the infernal into corrosion and sell the another one too. Build some jinx around to boost the damage. May lose quite an amount of life in the last 5 rounds.