the last battle was the hardest for me, after 5-6 monsters change Finley won it.
My tip is:
Take as much poison eater as you can.
The first 4 monster should be poison eater and most importent one when purify.
The first one first attack is poison gas, secound attack poison massacre, use it for your advantage, after he use poison gas use purify and then attack him with poison eater as he will also be poison.
His 3,4,5 attack are summen, ignore them continue to attack only him, when his dead deal with the others.
He will have 2 protector bei his side after the summon, but some attacks will still hit him.
Hope I helped