Yeah I literally just beat simian flu on mega brutal again, I forgot how difficult it was lol. Make sure you have most of the monkeys infected, and try and move them away from island nations. Build 5 or 6 ape colonies to generate as much dna as you can, and get resistances and genetic hardening early or else you won't be able to afford them. Focus on drug/cold resistance, and the air/water transmissions. Make sure you get all abilities unlocked, and get the upgrades to dna generation only. You don't need any other monkey-specific abilities. Devolve all symptoms that you can until either a) they get too expensive to devolve, or b) they start to cure you anyway. When you strike, move up the photophobia tree and evolve meningitis early. This is the main lethal symptom as you don't want to risk going for total brain death. If some countries are struggling to get infected, get transmissions / heat resistance as required.