Spread to undiseased cells right at the start. Keep going until you have every cell not occupied. Then spam their cells, and you win. Just get the highest reproduction rate possible and you win the game.
Btw, Do a BUNCH of missions later on for 100k each one.
I mean, my keyboard is litterally 1/2 broken, but when you have 3 MILLION gold, you can afford it. (5k each Elite Chivalry are my favorites...300dps!)
When you have 50,000 gold...just go. buy every potion in the shop, use your heal spell which you should've gotten from grinding, use health potions, etc. It's not even a challenge.
The cheap strategy to beat this game. Use the wing on Floor 18. Keep using it before you get to floor 19 so you can keep getting artifacts. Money also accumulates. Once you have a shield Sword and other misc Artifact Equips, you'll also have a large amount of money. So continue whenever you're ready. When you can buy every potion and everything in the shop that will improve your game, there's a high chance this game won't be a challenge anymore.
Just don't waste anything.